>>17760172Kecleon.
252+ Atk Life Orb Protean Kecleon Sucker Punch vs. 0 HP / 0 Def Mega Mewtwo Y: 390-460 (110.4 - 130.3%) -- guaranteed OHKO
252+ Atk Life Orb Protean Kecleon Sucker Punch vs. 0 HP / 0 Def Mewtwo: 320-377 (90.6 - 106.7%) -- 43.8% chance to OHKO
Even if Mewtwo survives, any psychic attacks will be useless. If you are fighting against Mega Mewtwo X you will have to use Shadow Sneak first and then predict the opponent's next move.