Toxapex @ Black Sludge
Ability: Regenerator
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature
Scald
Toxic
Recover
Haze
There's not much to say here, Toxapex is by far the best counter to Mega Scizor. Its typing, bulk, access to Haze, as well as being able to fish for Scald burns completely...