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This is mmx calc: 252 Atk Choice Band Adaptability Mewtwo-Mega-X Photon Geyser vs. 252 HP / 252+ Def Giratina in Psychic Terrain: 418-492 (82.9 - 97.6%) -- 68.8% chance to OHKO after Stealth Rock
Here are others without even hazards :
252 SpA Choice Specs Mewtwo-Mega-Y Psycho Boost vs. 252 HP / 252 SpD Giratina in Psychic Terrain: 486-573 (96.4 - 113.6%) -- 81.3% chance to OHKO
(fuck this comment limit tbh)