252+ Atk Pure Power Mega Medicham Double-Edge vs. 252 HP / 164+ Def Mew: 180-213 (44.5 - 52.7%) -- 82.4% chance to 2HKO after Stealth Rock and Leftovers recovery
252+ Atk Pure Power Mega Medicham Double-Edge vs. 252 HP / 252+ Def Mew: 168-198 (41.5 - 49%) -- 16.4% chance to 2HKO after Stealth Rock and Leftovers recovery
4 SpA Mew Hyper Voice vs. 0 HP / 0 SpD Mega Medicham: 145-172 (55.5 - 65.9%) -- guaranteed 2HKO
Use bold instead of impish and max out the defenses and you heve your counter. Hyper voice even goes past substitute (wich I've seen a couple times on the ladder)
Another good one would be sigilyph
252+ Atk Pure Power Mega Medicham Double-Edge vs. 252 HP / 252+ Def Sigilyph: 96-114 (27.5 - 32.7%) -- guaranteed 4HKO
252+ Atk Pure Power Mega Medicham Psycho Cut vs. 252 HP / 252+ Def Sigilyph: 87-102 (25 - 29.3%) -- guaranteed 4HKO
4 SpA Sigilyph Shadow Ball vs. 0 HP / 0 SpD Mega Medicham: 136-162 (52.1 - 62%) -- guaranteed 2HKO
252+ Atk Pure Power Mega Medicham Double-Edge vs. 252 HP / 252+ Def Mew: 168-198 (41.5 - 49%) -- 16.4% chance to 2HKO after Stealth Rock and Leftovers recovery
4 SpA Mew Hyper Voice vs. 0 HP / 0 SpD Mega Medicham: 145-172 (55.5 - 65.9%) -- guaranteed 2HKO
Use bold instead of impish and max out the defenses and you heve your counter. Hyper voice even goes past substitute (wich I've seen a couple times on the ladder)
Another good one would be sigilyph
252+ Atk Pure Power Mega Medicham Double-Edge vs. 252 HP / 252+ Def Sigilyph: 96-114 (27.5 - 32.7%) -- guaranteed 4HKO
252+ Atk Pure Power Mega Medicham Psycho Cut vs. 252 HP / 252+ Def Sigilyph: 87-102 (25 - 29.3%) -- guaranteed 4HKO
4 SpA Sigilyph Shadow Ball vs. 0 HP / 0 SpD Mega Medicham: 136-162 (52.1 - 62%) -- guaranteed 2HKO