252 Atk Mega Mewtwo X Earthquake vs. 252 HP / 56+ Def Primal Groudon: 170-200 (42 - 49.5%) -- guaranteed 3HKO
252 Atk Mega Mewtwo X Low Kick (120 BP) vs. 252 HP / 56+ Def Primal Groudon: 153-180 (37.8 - 44.5%) -- guaranteed 3HKO
Earthquake is important to damage Klefki and Aegislash but...