Rowlet is the Grass-type starter. It can be received from Hala in Iki Town.
The most popular starter of the Alola region ever since long before the release of the seventh generation: a tanky Grass-type, fortunately set in a region in which Fire-types and Ice-types are essentially extinct. This works very much in Decidueye's favour, especially in its earlier evolutionary stages, where the double Ice weakness would have turned out even nastier. Much like many Grass-type starters before it, Decidueye is by no means a glass cannon or easy to use; as its typing would suggest, it also struggles in a number of matchups, being hit hard by the fairly common Flying-type moves as well as suffering the super effective Sucker Punch creep after hitting its final evolutionary stage.
For these reasons, one could argue that Rowlet is the worst of all starters. In a relative sense, that could be right. However, with the likes of Incineroar and Primarina as its competition, this only says so much about its capabilities in battle. Decidueye has a rare combination of good bulk on both sides of the spectrum, great attacking power on both sides of the spectrum, and not-so-bad Speed in a region where countless species are slower than it. It is a near-perfect Pokémon on paper, that is only really hindered by its weaknesses in practice. Decidueye is not a straightforward, press-button-and-win kind of Pokémon; in almost every matchup it is used in, it requires strategy and good management to perform well, but it decidedly rewards patient players in their game.
Hau battle #1 (Iki Town): Leafage is a clean 2HKO; there is not much Popplio can do.
Hau battle #2 (Iki Town): Leafage now oscillates between 2-3HKO against Popplio, but the matchup is still easy due to Popplio's low Pound damage. The fight may be slightly more complicated against Pichu, which can paralyse with Static; however, Thunder Shock cannot KO anywhere below five hits and Leafage still manages to 3HKO it.
Ilima (Hau'oli City, Normal-type): Rowlet easily beats Yungoos: its Tackle is only as powerful as Leafage after one Leer, and Leafage itself is a 4HKO, so Yungoos will not have enough time to inflict significant damage. Smeargle is more difficult: its Technician-boosted Ember reaches a damage output that is higher than Leafage, but Smeargle also outspeeds, and if Yungoos has used Leer before it will also take advantage of Rowlet's Defense drop with Tackle. If Rowlet must fight Smeargle, tread with caution and have healing items ready.
Totem Gumshoos/Raticate (Verdant Cavern): Rowlet is at a decisive disadvantage against both of the Totem Pokémon, which averagely 3-4HKO with Bite (Gumshoos is more likely to do so, due to also having Super Fang), and Rowlet's best move is Leafage; even if it already knows Razor Leaf, the base power is reduced against multiple opponents, and the ally Yungoos or Rattata can lower Rowlet's Defense further with Leer and Tail Whip respectively, making this matchup too dangerous all around.
Hau (Route 3): Pikachu's Electro Ball now outpaces Leafage, both scoring a 3-4HKO with Pikachu being faster; Rowlet may win if it already knows Razor Leaf, which is a certain 3HKO, or by using Breakneck Blitz on the first turn and then Leafage. Razor Leaf also makes beating Popplio easier, since it knows Baby-Doll Eyes and the move can easily lengthen the fight.
Hala (Iki Town, Fighting-type): Rowlet's Peck easily 2HKOs Mankey and 2-3HKOs Makuhita. The move is a 4HKO against Crabrawler, which can inflict up to around one third of Rowlet's health with All-Out Pummeling; however, it is recommended to have an X Defend and/or healing items if Rowlet must solo the fight, because Hala is likely to heal and, against Flying-types, Crabrawler usually starts with Leer. An All-Out Pummeling from -2 Defense is bad even for Rowlet; compensating the Defense drops is a safer option before continuing to Peck it.
Ilima (Hau'oli City, Normal-type, optional): Rowlet's matchup against Smeargle is better, since it can now profit of Razor Leaf to 3HKO; Smeargle can 3HKO with Ember as well, though Rowlet will pull through if it scores a critical hit or is healed in between. Breakneck Blitz can help: combined with a critical Razor Leaf, it can 2HKO Smeargle. However, Gumshoos' Breakneck Blitz is absolutely devastating for an unevolved Rowlet, and even its regular Tackle deals much more damage than Rowlet's STAB.
Hau (Paniola Town): Pikachu's Electro Ball is significantly less powerful against Dartrix, which now wins the matchup by 3HKOing with Razor Leaf. If Dartrix is still anywhere above half health when Brionne comes out, it can easily solo the fight, since even Breakneck Blitz does not even manage to inflict one third of Dartrix's health in damage, while Razor Leaf hovers around the 2-3HKO without critical hits.
Gladion (Route 5): Dartrix may technically just barely manage to beat Zubat with Peck if it outspeeds, but Wing Attack hurts about as much, and trying to fight Zubat with Dartrix is not worth it. Dartrix's health is best saved for Type: Null, which Dartrix can defeat by spamming Razor Leaf and healing as needed, since Type: Null's Tackle deals more damage. Breakneck Blitz will make either matchup more favourable, and is also best saved for Gladion's ace.
Totem Wishiwashi (Brooklet Hill): Despite Dartrix's type advantage, this fight is not easy. Under the rain and with Helping Hand in effect, Totem Wishiwashi's Water Gun is almost as powerful as Dartrix's Razor Leaf, or more powerful if Growl was used once or more. In addition, leading with Razor Leaf is not wise; if Totem Wishiwashi's health drops below half as a result of a critical hit, it will call Alomomola first, and Wishiwashi only later, making Alomomola spam Heal Pulse on it. The fight is best opened by using a Dire Hit or, at least, equipping Dartrix with the Scope Lens, which will significantly improve its chances in this battle. Dartrix should preferably use Leafage rather than Razor Leaf if the ally Wishiwashi is still alive, as this will prevent Totem Wishiwashi from calling Alomomola. With two Razor Leaf critical hits, Totem Wishiwashi will go down. A possible alternative is to increase Dartrix's friendship to the maximum and teach it Grass Pledge in Hau'oli City; this will lead to a much easier 2HKO, even without critical hits.
Battle Royal (Royal Park): Dartrix is not the ideal pick for this set of fights, because Rockruff cannot be OHKOed by Razor Leaf and its Rock Throw is almost a 2HKO; on top of the damage Dartrix may take from Type: Null's Tackle and/or Brionne's Breakneck Blitz, a single unfortunate turn may be enough to kill the owl. In addition, although Razor Leaf inflicts massive damage to Rockruff, it has Protect and can easily dodge the damage; Grass Pledge OHKOs Rockruff for certain, if Dartrix knows the move, but it still cannot bypass Protect.
Totem Salazzle (Wela Volcano Park): Dartrix must not partake in this fight. Totem Salazzle's Flame Burst is a 3HKO without even counting the Petaya Berry, it can use Toxic, and the ally Salandit all know Venoshock. Grass moves almost cannot scratch them at all.
Totem Lurantis (Lush Jungle): Totem Lurantis would not be problematic for Dartrix, since it can only 3HKO with X-Scissor (not guaranteed) while Dartrix 2-3HKOs with Pluck, but the Totem's allies are horrible for Dartrix to fight against. Trumbeak outspeeds Dartrix and knows both Pluck and Rock Blast, while Castform is nearly certain to OHKO Dartrix with Weather Ball after putting up Sunny Day. If Totem Lurantis' health is below two thirds, it will call Castform, which makes the fight impossible for Dartrix. Dartrix should only be switched in if the Totem is the only Pokémon left alive.
Plumeria (Akala Outskirts): Dartrix is too disadvantaged against both Golbat and Salandit, which outspeed and 2HKO it.
Olivia (Ruins of Life, Rock-type): Razor Leaf can unfortunately only 3HKO Nosepass, while Grass Pledge is a 2-3HKO, but there is nothing else Dartrix can do in this matchup. Boldore's Rock Blast can even OHKO with five hits and no critical hits, if the damage roll is high enough, while Lycanroc 2HKOs with a combination of Continental Crush and regular Rock Throw and can only be 2HKOed by a Dartrix with Grass Pledge and good Special Attack which still is at full health when Lycanroc is sent out. If Dartrix goes against the ace, Bloom Doom is unusable in this matchup: while a fully healthy Dartrix with the Eviolite is at a risk of being OHKOed by Continental Crush with below-average Defense, a Dartrix holding the Grassium Z cannot take a Continental Crush at all; the lack of Eviolite also leaves it open to taking harder hits from the rest of Olivia's team.
Lana (Konikoni City, Water-type, optional): Dartrix has better chances against Lana if faced with the Eviolite. While Shellder must be avoided due to Skill Link Icicle Spear, an OHKO in most cases, Dartrix's higher Speed makes the matchup viable if it knows Grass Pledge, a net OHKO against Shellder. Chinchou can be 2HKOed with Razor Leaf and Araquanid is, on average, 3HKOed by Pluck. Even its Bug Bite does not do much to Dartrix at all.
Kiawe (Royal Avenue, Fire-type, optional, Sun only): Dartrix can beat Fletchinder with Pluck, if it holds the Eviolite; Pluck outdamages both of Fletchinder's STABs. To do this, however, Dartrix cannot be in the lead, as Growlithe's Intimidate will otherwise cut down the power of Pluck. Marowak knows Flame Wheel, which is too strong.
Mallow (Lush Jungle, Grass-type, optional, Moon only): Pluck is a 2HKO against Phantump and Steenee, and a 3HKO against Shiinotic. Dartrix rules the place.
Nihilego (Aether Paradise): Nihilego's Venoshock kills Dartrix in two turns, even without poisoning and with the Eviolite attached. Do not fight Nihilego.
Hau (Malie City): Dartrix can defeat both Raichu and Brionne, respectively 3HKOing and 2HKOing with Grass Pledge; this only works with the Eviolite and some healing, as Raichu's Psychic will otherwise deal too much damage, also being a 3HKO. The matchup against Raichu changes with Bloom Doom, which can 2HKO paired with a regular Grass Pledge, but also leaves Dartrix more vulnerable due to the lack of Eviolite. Flareon must be avoided, as it easily beats Dartrix with Fire Fang.
Totem Vikavolt (Hokulani Observatory): Dartrix is not good against Totem Vikavolt, which resists its best available STAB and is neutral to the other, thanks to its typing. By contrast, Spark is a 4HKO against Dartrix, and can even 2HKO if Charge is used, not counting the damage Dartrix can take from the ally Charjabug (if present). Overall, unless no better answers than heal stalling are available, Dartrix should not be used.
Guzma (Malie Garden): An Eviolite Dartrix might win against Golisopod with Protect and Pluck; Protect shields it from the likely First Impression on turn one, and it takes Golisopod two Swords Dances to barely have 2HKO chances with Razor Shell, while Dartrix 3-4HKOs with Pluck and is nearly guaranteed to take Golisopod to Emergency Exit range before it can set up that much. Ariados is also trivially 2HKOed by Pluck, and does not threaten Dartrix due to its lack of Poison moves. Regardless, Dartrix should not fight it in low health (below 25%), since Ariados knows Sucker Punch.
Totem Mimikyu (Thrifty Megamart): Dartrix is easily shot down by a critical Play Rough from Mimikyu and either Night Shade or Shadow Ball from its allies, and cannot deal much damage in return, even with Shadow Claw. This is also assuming the Eviolite, as a non-Eviolite Dartrix is even more vulnerable. Decidueye is even more disadvantaged, due to the acquired Ghost weakness; while Spirit Shackle would otherwise 2HKO Mimikyu, Mimikyu outspeeds and can even OHKO with Shadow Claw, thanks to its aura boost. Even Sinister Arrow Raid is useless against Mimikyu's Disguise.
Plumeria (Route 15): Decidueye can 2HKO Golbat with Spirit Shackle, but only if Golbat's Air Cutter does not score a critical hit, which would bring the move from 3HKO to 2HKO range; alternatively, Sinister Arrow Raid OHKOs Golbat. Salazzle 2HKOs Decidueye with Flame Burst, and must be avoided.
Guzma (Shady House): It is best for Decidueye to lead with Protect, avoiding a possible First Impression from Golisopod. After the first turn, Acrobatics will 2-3HKO, meaning that after one or two hits (the likelihood depends on Decidueye's Attack stat, and is almost perfectly even at a value of 95) Emergency Exit will activate. Both Bloom Doom (off Grass Pledge) and Sinister Arrow Raid are also guaranteed to deal more than 50% to Golisopod; Bloom Doom is stronger, due to Golisopod's lower Special Defense. Decidueye should then preferably switch against Ariados, since it is now weak to Sucker Punch, although a healthy Decidueye can 2HKO Ariados with the move, even if this will take away most of its health. A Decidueye above one third of its health that knows Protect can then finish off Golisopod when it comes back out, after shielding itself from First Impression once again.
Gladion (Aether House): Decidueye must avoid Golbat's Acrobatics at all costs. It can U-turn in front of Sneasel, whose most damaging move is Feint Attack, though even that 2HKOs and will thus take most of Decidueye's health if U-turn is used; U-turn also nearly kills Sneasel, however. Decidueye's best matchup is against Type: Null, since it can outdamage Pursuit with Grass Pledge, 3-4HKOing it; regardless, Pursuit is a 4HKO as well, so this matchup is likely to require some healing unless Bloom Doom is used, since the move can 2-3HKO together with regular Grass Pledge.
Nanu (Malie City, Dark-type): Decidueye's Spirit Shackle is a 2-3HKO against Sableye, though Sableye's Shadow Ball chips away over half of Decidueye's health in turn, making the matchup dangerous without resorting to Sinister Arrow Raid. Grass Pledge also OHKOs Krokorok, but Decidueye must either outspeed or be at full health to take a Crunch from it beforehand. Persian must be avoided, its Black Hole Eclipse starts at a base power high enough to one-shot.
Faba (Aether Paradise): Spirit Shackle is an easy 1-2HKO against Hypno.
Faba (Aether Paradise, tag battle with Hau): Decidueye can easily two-shot Faba's Slowbro with Grass Pledge, but that should happen only after it OHKOs Ledian with Acrobatics. Ledian's Reflect and Light Screen make Faba's Pokémon significantly more dreadful, and Decidueye's good Special Defense enables it to take two or three Psychics from Slowbro either way. Bruxish's Speed is 88; it OHKOs with Crunch but also gets OHKOed by Spirit Shackle, so if Ledian has already been defeated and Decidueye can outspeed, it will win the matchup. Hypno, once again, is 1-2HKOed.
Guzma (Aether Paradise): Decidueye should use Protect first, to avoid getting hit by Golisopod's First Impression. It can 2-3HKO with Acrobatics afterwards; Razor Shell is a 3-4HKO without any Swords Dances, but even attempts at using Swords Dance will be foiled by Emergency Exit, and the ability is also guaranteed to activate with either of Decidueye's STAB Z-moves. It is recommended to switch if the next Pokémon is Ariados, as its Sucker Punch 2HKOs Decidueye and has increased priority; Masquerain's Air Slash can also deal up to around 85%, and Acrobatics only has a chance to OHKO it, not a certainty. Decidueye also needs a Speed of 86 or higher to win against Pinsir, since both Acrobatics and Pinsir's X-Scissor are 2HKOs. Sinister Arrow Raid unfortunately does not help, failing to OHKO Pinsir just barely.
Lusamine (Aether Paradise): Decidueye's Spirit Shackle slightly outdamages Clefable's Moonblast, though both are 3-4HKOs, so which Pokémon wins is more a matter of luck; in addition, as Clefable tends to use Metronome, the matchup against it could go very well or very badly, depending on sheer luck. In any case, Clefable cannot OHKO Decidueye from full health, as even strong super effective moves like Overheat are 2HKOs at worst; Sinister Arrow Raid is the most sure-fire way of 2HKOing Clefable together with Spirit Shackle. Acrobatics 2HKOs Lilligant easily; without an item, though, Decidueye might still get confused by its Teeter Dance. Going up against Mismagius is risky: although Decidueye's Spirit Shackle is a guaranteed OHKO, Mismagius' Shadow Ball has an OHKO chance that is usually low, but varies with Decidueye's bulk. Beating Milotic and Bewear is trivial: Milotic's Hydro Pump barely scratches the 3-4HKO range and Flail has no effect on Decidueye, while Bewear can only hit with STAB, and Decidueye is immune to both of them.
Hapu (Vast Poni Canyon, Ground-type): Decidueye must not fight Dugtrio, whose Sucker Punch is an easy 2HKO; Leaf Blade cannot OHKO it, though Sinister Arrow Raid or Bloom Doom (running off Leaf Blade) will do so. Gastrodon is OHKOed easily instead, and Leaf Blade also 2-3HKOs Flygon, which has an entirely special moveset and can at best shoot for a 4HKO with Dragon Breath. Decidueye must know Energy Ball to win against Mudsdale, which has Stamina as its ability; two Leaf Blades would not KO it, and its Heavy Slam is a 2HKO, which can potentially OHKO with a critical hit. While it is unlikely for Mudsdale to use Tectonic Rage, the move is even stronger than Heavy Slam, being still a 2HKO but strong enough to always OHKO on a critical hit. The only way for Decidueye to be completely safe against Mudsdale is to save its Z-move usage and fire off an Energy Ball-based Bloom Doom, which is guaranteed to OHKO.
Totem Kommo-o (Vast Poni Canyon): The Totem is difficult to beat, as it 2HKOs with Clanging Scales and is guaranteed to OHKO with a critical hit. Sinister Arrow Raid has an OHKO chance only at -1 Defense, which comes after two Clanging Scales drops due to the Totem's aura; at -2, the move is guaranteed to OHKO. Decidueye's only option is that of stalling the Clanging Scales PP until all the totem has left to hit it is Flash Cannon, which makes it lose the matchup due to the multiple stat drops. Scizor's Pursuit is problematic only below 20% health if Decidueye stays in, and Clanging Scales will not deal enough damage to take Decidueye's HP in the KO zone unless either its Defense or Special Defense are significantly below average.
Lusamine (Ultra Space): Spirit Shackle 3HKOs Clefable, which can also be 2HKOed with a combination of Sinister Arrow Raid and Spirit Shackle; the matchup is not entirely safe, due to Clefable's access to Metronome and potential 2HKOs if strong super effective moves are pulled, though Decidueye will not be OHKOed by any one hit unless it is a critical hit from a strong super effective move. Decidueye may also opt for Swords Dance on the first turn, which turns Spirit Shackle into a 2HKO and the Z-move into an OHKO, not changing the needed amount of turns to win; two Swords Dances will do the same, as well. However, Swords Dance setups against Clefable are not recommended, since Mismagius is likely to switch in afterwards and its Shadow Ball has remarkable chances to OHKO Decidueye. Lilligant can only be OHKOed by Acrobatics after a Swords Dance (if Decidueye is not holding an item) or Spirit Shackle after two; Sinister Arrow Raid OHKOs even without setups, though both choices are not risk-free as Swords Dance will encourage Lilligant to use Teeter Dance, causing massive confusion recoil, while a lack of setups requires Decidueye to use its Z-move to win, since Petal Dance would otherwise 2HKO despite Decidueye's good bulk and resistance. Milotic and Bewear are easy to beat; Milotic is 2HKOed by Leaf Blade, or OHKOed by Leaf Blade-powered Bloom Doom, while Bewear has no moves that can damage Decidueye outside of Pain Split, which is easy to work around.
Solgaleo (Altar of the Sunne, Sun only): No. Decidueye is outsped and 2HKOed by Crunch, and cannot OHKO Solgaleo even with Sinister Arrow Raid.
Lunala (Altar of the Moone, Moon only): Decidueye is destroyed by Moongeist Beam. Again, no.
Gladion (Mount Lanakila): Crobat and Silvally can kill Decidueye easily with Acrobatics and Multi-Attack respectively, and must be avoided at all costs. Low Sweep OHKOs Weavile, but Decidueye may also be OHKOed by Night Slash; although this is not highly probable, Decidueye with lower-than-average Defense will not make the cut, and critical hits will kill regardless of nature. Decidueye's matchup is good only against Lucario, which can be 2HKOed by Low Sweep, potentially also being outsped on the second turn.
Hau (Mount Lanakila): Spirit Shackle OHKOs Raichu; Primarina is 1-2HKOed by Leaf Blade, though the OHKO is only possible (and not guaranteed) with the Expert Belt. Bloom Doom does OHKO Primarina, instead. Decidueye must not come close to Flareon, which knows Flare Blitz. Komala can be OHKOed with Bloom Doom, but the likelihood depends on Decidueye's Attack and should this not happen, a switch-out will be in order, else Komala is highly likely to finish off Decidueye with Sucker Punch.
Elite Four Hala (Pokémon League, Fighting-type): Hariyama may be defeated either with an Acrobatics 2HKO, or a Swords Dance and then Acrobatics OHKO. Although Swords Dance would generally be the preferred option, it cripples Decidueye against Primeape, which knows Punishment and can finish Decidueye after Hariyama's Knock Off; Acrobatics OHKOs Primeape regardless of setups, if Decidueye's health is still high enough to take Punishment (above 55% without Swords Dance, full or close to full otherwise). Bewear's Fluffy and Brutal Swing make the matchup impossible for Decidueye; Poliwrath is 2HKOed by Leaf Blade, which has OHKO chances with the Expert Belt, else Poliwrath may be OHKOed with Bloom Doom. This, however, compromises the Acrobatics sweep due to the necessity of holding an item. Decidueye must avoid Crabominable anywhere below +2 in Attack: unless Acrobatics nets the OHKO, Crabominable will do so with Ice Hammer.
Elite Four Olivia (Pokémon League, Rock-type): Relicanth is an excellent opportunity for Decidueye to set up, since even its strongest move Ancient Power is weak against the starter's very good special bulk. Unless Decidueye's Attack is far below average, a single Swords Dance is sufficient to OHKO Relicanth, Carbink and Lycanroc, and 2HKO Golem and Probopass, which cannot be OHKOed either way because of Sturdy. The greatest threats to Decidueye's health are Golem's Rock Blast and Lycanroc's Crunch; in order to survive them with certainty, Decidueye's health must be 75% or higher in the first case, and 65% or higher in the second, which covers even against a five-hit Rock Blast. Probopass is best for healing purposes, as Carbink can ruin Decidueye's sweep by setting up Reflect.
Elite Four Acerola (Pokémon League, Ghost-type): Decidueye is better off not leading, since Sableye may or may not outspeed - depending on whether Decidueye's Speed is lower or higher than 109 - and unlike Sableye, Decidueye is weak to Shadow Claw. Sinister Arrow Raid can OHKO it, but it is not worth it. Drifblim can be 2HKOed with Spirit Shackle or OHKOed with the Z-move; Sinister Arrow Raid is recommended if Drifblim has acquired multiple stat boosts, especially with Focus Energy thrown in the mix, since it can Baton Pass them to more dangerous teammates. Like Drifblim, Dhelmise is 2HKOed by Spirit Shackle or OHKOed by the Z-move, though in this case the latter is not necessary, since Decidueye outspeeds and can take a Shadow Ball. Given this, if faced against Drifblim or Dhelmise, Decidueye should put up Swords Dance the first turn and then attack on the second, OHKOing even without the Z-Crystal. If its health is full, Decidueye can beat even Froslass, since it just barely survives Blizzard even with the highest damage roll - unless its Special Defense is below average - and OHKOs for certain with Spirit Shackle. If Decidueye's Z-move is still available, its best possible use is against Palossand; it nets an OHKO, preventing it from unleashing an otherwise deadly Never-Ending Nightmare.
Elite Four Kahili (Pokémon League, Flying-type): Even though Skarmory lacks Flying STAB entirely, its good physical bulk makes it hardly vulnerable to Decidueye's best moves; only Shadow Ball can 2HKO, and Skarmory's Speed allows it to put up at least two layers of Spikes in the process, a serious handicap for most teams. Setting up Swords Dance against it would not help, either: Mandibuzz's Punishment would OHKO it flat, and Mandibuzz is unbeatable without Attack boosts; Oricorio's Revelation Dance is also very strong, and while it does not OHKO, only a fully healthy Decidueye can take one and then OHKO back with Sinister Arrow Raid. Alternatively, Decidueye may beat Crobat with Sinister Arrow Raid and Sucker Punch combined, which bypass Crobat's Air Slash 2HKO; this will only happen if Crobat does not use Supersonic on the Sucker Punch turn, however. Lastly, Toucannon destroys Decidueye with Beak Blast.
Professor Kukui (Pokémon League): Lycanroc is OHKOed by Leaf Blade. Ninetales is highly likely to be OHKOed by Sinister Arrow Raid, but its Blizzard comes close to an OHKO; specimens with lower-than-average Special Defense may be OHKOed by it even without critical hits. Braviary one-shots with Brave Bird, and Snorlax can 2HKO with Crunch, leaving Decidueye at a severe disadvantage. Even the matchup against Magnezone is hard to win, since Flash Cannon is close to the 2HKO range; only Light Screen support can aid Decidueye which, having the Screen on at all times, may manage to eventually KO Magnezone with Spirit Shackle. Sinister Arrow Raid makes the job quicker, as well. Incineroar should never be approached.
Tapu Koko (Ruins of Conflict): Since Decidueye resists Tapu Koko's only two damaging moves besides Nature's Madness, it can easily 2-3HKO with Leaf Blade or even 1-2HKO with Sinister Arrow Raid.
Post-Game: Decidueye's matchups against the Ultra Beasts are generally fairly positive, though it maintains a disadvantage against several challengers in the League rematches.
Rowlet starts with Tackle, Leafage and Growl as its moves. Its base Grass STAB is good, and Flying STAB comes shortly after at level 8, as Peck. Rowlet then obtains further coverage at 11, through Astonish; this move is not exceedingly powerful, but it will generally be better than Tackle. At level 15, Leafage finally gets upgraded by Razor Leaf just before evolution. Dartrix then learns Foresight at level 19, but given its future Ghost-typing, the move is entirely useless. A much better moveset addition is Pluck, learned at level 24. Nuzlockers with clauses that limit or forbid healing will also want Synthesis, accessible starting at 28; however, in runs without any such healing restrictions, Synthesis will generally be a waste of a slot. Fury Attack, at level 33, is useless regardless of nuzlocke rules. Decidueye then learns Spirit Shackle upon evolution, its best available Ghost move on par with Shadow Ball. At level 38, Decidueye gets Sucker Punch; while not bad itself, the priority relies upon the opponent using an offensive move, and Dark coverage largely overlaps with Ghost coverage, rendering Dark moves redundant more often than not. At level 44, instead, Decidueye's long-standing Grass STAB finally obtains another upgrade with Leaf Blade. Another somewhat useless move comes at level 49 in the form of Feather Dance; further coverage comes at level 55 with Brave Bird instead, though Decidueye's base HP are not sky high, so the move should be used sparingly and with due caution. Lastly, for special sets and if Decidueye ever gets that far, Nasty Plot becomes accessible at level 60.
TM coverage offers further STAB options, particularly in terms of special moves, which Decidueye's level-up moveset is lacking. Energy Ball and Grass Knot are both viable alternatives; Energy Ball should be preferred in most matchups, as its base damage is more reliable. Shadow Ball provides obvious Ghost STAB, instead; Shadow Claw can be learned before Spirit Shackle comes around, as well. The physical equivalent of Nasty Plot, Swords Dance, is also available; this makes Decidueye a well-rounded sweeper, capable of great offensive presence on both sides of the spectrum. Most other TM moves this line learns are exclusive to the final evolutionary stage: Acrobatics is one example, a good replacement for Brave Bird, though it comes at the expense of sacrificing the held item; U-turn works well on sets that do not rely on setup moves, though it does not have great synergy with Swords Dance. Low Sweep may have been an interesting option, but its base power is too low to matter by the time Rowlet reaches its final evolution. Light Screen is a viable defensive option; it works especially well in tandem with U-turn in a setup-and-run scenario.
A number of Z-moves are available for Decidueye. Breakneck Blitz will help in the very early game, but it will also be quickly outclassed by STAB moves, particularly Grass Pledge. Bloom Doom is great and offers good coverage, even against Pokémon that are neutral to Grass; running off Grass Pledge and, later on, even Leaf Blade, it can turn a few matchups upside down. For Pokémon that resist Grass moves, Sinister Arrow Raid will get the job done, often scoring OHKOs or near-OHKOs unless the enemy also sports a Ghost resistance or immunity.
Rock-types: Rock-types cover for one of the two most commonly seen weaknesses of this line, especially in the early game; in addition, Rock-types usually have high Defense, and while Decidueye's Defense is not bad, having a proper physical wall on the team is often helpful.
Dark-types: The late game is ripe with threats to Ghost-types. Dark-types complement them best, and Decidueye is more than capable of covering for a Dark-type teammate's weaknesses, creating good team synergy.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Rowlet Line Ratings
What Nature do I want? Adamant is the best possible nature for Decidueye's movepool. Modest can be excellent, as well, but only in the later portions of the game, when the good special moves become available. Since Decidueye's stats can all be used in some manner, any nature will have its pros and cons; it is simply a matter of suiting its moveset to its potential.
At what point in the game should I be evolved? Dartrix should be evolved shortly after landing on Akala Island, potentially before battling Sina or Dexio, as it can be an asset against Dexio in particular. Decidueye should be evolved anywhere between the arrival in Malie City and right before or right after the battle against Totem Mimikyu.
How good is the Rowlet line in a Nuzlocke? For most matchups, neither good nor bad. Decidueye does not particularly shine in any battle, though it is also capable of taking down a few Pokémon in each. It must be used with forethought and consideration; if well managed, it can be a valuable asset, though by no means an all-round problem solver.