Mimic could have defended against the onslaught of attacks easily enough, if Griffin hadn't practically dropped like a stone. he paused for a few seconds, dropping the light box on a curse. 

And still, he defended against the magical attack from Kat, defended against the electrical attack from Makoto, but wasn't quite quick enough as Phoebe darted in, what appeared to be a flower in her hands. 

"You made a mistake" She said almost cheerfully, the plant was a natural, and quite potent, sleep inducer. "You can't copy the  natural immunity I have, because it's in my genes. Powers, yes, immunity no" 

Perhaps it was the combination of attacks, or perhaps Mimic had simple reached his limit, but he stumbled back, away, flinging the flower away, but the damage was done.