Not going to lie, none of this generation's three starters are all that great. Decidueye is a balanced attacker, with okay Special Defense, but due to the power creep (especially with his lackluster base speed of 70 which is on par with Samurott), he'll probably never make it to OU. How sad.
Positives-
+Base 107 Attack is decent.
+Spirit Shackle is a fairly good signature move, with base 80 power, and it traps foes, too. It also allows Decidueye to use Sinister Arrow Raid if it so wishes.
+Quite bulky on the special side with base 100 Special Defense.
+Its unique ability, Long Reach, allows it to use moves like Leaf Blade without the worry of hitting something like Flame Body, Static or Effect Spore.
+Looks cool.
Negatives-
-Base 70 Speed puts it in the Dead Man's Land when it comes to speed- too slow to function IN Trick Room, too slow to function OUTSIDE Trick Room.
-Its base Defense of 75 and base HP of 78 isn't doing it any favors, either, making a bulky set kind of out of the question.
-Fairly shallow movepool. It doesn't really get much of use outside of Leaf Blade, Sucker Punch, Spirit Shackle, Brave Bird and U-Turn.
-Despite getting Nasty plot, a Special Set is just flat-out unusable. The best Special Moves it gets are Shadow Ball and Energy Ball, and that's it. So its base 100 Special Attack is useless. Literally the only move other than those that it gets that can be remotely considered is Hidden Power. This means burns are fatal for the owl.
With that out of the way, here's the set.
I had to become someone else, something else... I became... the Green Arrow.
Decidueye @ Decidium Z/Life Orb
EVs and Nature: Adamant or Impish Nature.
Ability: Long Reach when it becomes legal.
252 HP, 132 Atk, 128 Def.
-Spirit Shackle
-Leaf Blade
-Sucker Punch
-Synthesis/Roost/Brave Bird/Swords Dance/U-Turn.
Decidueye is a Pokemon who has a limited movepool but would be able to make use of it to devastating effect were it not for its lackluster stats. In particular its base 70 speed is a big problem, and no speed investment can fix this. Decidueye only reaches 262 speed with maximum investment (level 100, perfect speed IV, boosting nature, 252 EVs), so it's just not worth it since certain OU threats will outspeed and destroy it anyway. Pretty much any Garchomp variant worth a damn will absolutely destroy it. Decidueye shines in the lower tiers, though, where its good combination of ability and moves can shine, but not due to its stats. Decidueye should run bulky over speed since even in UU and lower, there are still 'mons that will reliably outspeed Decidueye and take it out. But with that being said, on with the moveset.
Spirit Shackle is Decidueye's Signature Move and a pretty good one, with base 80 power and a trapping effect, akin to Thousand Waves. Use it on a 'mon that Decidueye can threaten, and if it doesn't switch out, it's screwed as it will be trapped with Decidueye.
Leaf Blade is the obligatory secondary STAB. Good power, decent Crit Rate, you get the idea.
Sucker Punch allows Decidueye to deal with faster Ghosts and Psychics that would just Shadow Ball the owl into oblivion, like Gengar or Alakazam.
The last slot can be a number of things. Swords Dance allows Decidueye to boost its attack and make its hits HURT. U-Turn and Brave Bird grant extra coverage, and U-Turn has the added effect of maintaining momentum. Roost and Synthesis are for healing.
The item depends. If you want a one-shot wonder, you can give Decidueye the Decidium Z to have it use the Base 180 Power Sinister Arrow Raid, which should smash most less-than-bulky 'mons, like normal Charizards. If you want something a bit more damaging overall, Life Orb can boost Decidueye's damage output.
Countering and Checking Decidueye
Most things faster with a decently powerful STAB can just wipe the Owl out, as its defenses aren't that great. The only reason a Charizard would ever hesitate to come in on a Decidueye is if it's weakened or if the guy using a Charizard knows that Decidueye is carrying Decidium Z. Bulky Megazard X has nothing to fear from Decidueye and can render it useless through Will-O-Wisp, or just annihilate it with Flare Blitz, while taking low damage from Spirit Shackle. Arcanine and Heatran also work well in curbing the owl, as do most Dark-Types, as Decidueye doesn't have much to hit them with. Sharpedo can't take a Leaf Blade, but if it comes in safely, Decidueye MUST switch out or get annihilated by Crunch. Bisharp walls everything Decidueye can throw out, and can use either of Knock Off or Sucker Punch to do Decidueye in. Absol, too, can do so. In fact, there are very few Dark-Types that can't deal with Decidueye. Cacturne and Shiftry can't due to Brave Bird and U-Turn. Decidueye can't do anything to Hydreigon either, barely taking damage from anything barring U-Turn and one-shotting the sniper owl with Dark Pulse.
