Mirror-Mimic Slime

Evolve (Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)
Whenever a nontoken creature enters the battlefield under an opponent's control, create a X/Y green Mutant creature token, where X is its power and Y is its toughness.
It hungers for perfection in all it sees.