Mister Sinister, Mutagenuis

At the beginning of combat in your turn, put
a -1/-1 counter on up to one target creature.
{1}: Until your next turn, Mister Sinister, Mutagenius becomes a copy of target creature with a counter on it, except it's 7/5
and has this ability.
Visionary. Savior. Madman. It doesn't matter.
I am SINISTER.