UltimateWario
Smash Master
I was honestly thinking that Mewtwo transforms into Megatwo Y (because seriously, he's going to be in one way or another), then uses Psystrike, which causes tons of tiny shards of telekinetically-formed glass (see Psystrike's XY animation) to radiate out from his position. If you're within a certain range of Mewtwo when he does this move, you're dead. He then simply transforms back into regular Mewtwo.