Mega Mewtwo Y's high speed and special bulk let it pull of a sort of stall breaker set much like 5th and 6th gen's "stall two" set.
orch gave me the idea, I don't really know how though.
Mewtwo @ Mewtwonite Y
Ability: Pressure
EVs: 176 Spd / 252 HP / 80 SDef
Timid Nature
- Taunt
- Will-O-Wisp
- Psystrike
- Recover
EVs are pretty simple, after Mega Evolving you have enough speed to outrun Mega Gengar and whack it with a psystrike. The rest is invested into its HP and special defense to make it very bulky. For reference you can beat Palkia given its not getting any boosts from rain o_O.
252 SpA Lustrous Orb Palkia Hydro Pump vs. 252 HP / 80 SpD Mega Mewtwo Y: 192-226 (46.1 - 54.3%) -- 53.1% chance to 2HKO
Wrecked. You simply stall until you get a miss or just pp stall hydro pump entirely. Spacial Rend deals less damage and / or is worse. You then proceed to 2HKO with Psystrike whenever you get the chance or just burn stall. Assault Vest Palkia cry.
If you haven't used Stall Two before the idea is to Taunt the opponent to prevent them from healing, WoW acts like a psuedo Reflect that also sneaks passive damage in, Recover is obvious Recovery, and Psystrike smacks stuff / speeds up their death.
Light Screen was viable in previous generations but the ability to outspeed + OHKO Mega Gengar is too much to give up.
This thing is also probably the best thing to use if you wanna keep your hazards up. After Stealth Rock, Psystrike OHKOes ho-oh and this Mewtwo cannot be bypassed by any of the common Support Arceus (except Darkceus and maybe Ghostceus). Scizor has the best chance but it'll get burned in the process and rendered useless. Giratina-o is in a similar boat except Shadow Balls will actually hurt after its burned. It probably also single handedly gives quick stall a chance to thrive this generation.