I'm thinking about:
Morobareru @ Leftovers
Bold (+Def, -Atk)
252 HP, 56 Def, 100 SpAtk, 100 SpDef
Ability: Regeneration
~Spore
~Toxic
~Giga Drain
~Synthesis
Spore because that's one of the few moves that Mr. Mushroom uses well. Once Sleep Clause is activated, you spread Toxic around...