Mewtwo Lv. X is ... (drumroll thank you) ... not a broken card. 120 damage every two turns for a "stage one" is excellent. Immunity to other basics is awesome too (Why are you people messed up about his body?! Just read:
Poke-Body Psybarrier
Prevent all effects of attack, including damage, done to Mewtwo by your opponent's pokemon that isn't an Evolved Pokemon.)
A possible 140 hp from Snowpoint makes it hard to kill (despite the x2 weakness). But it is nevertheless not broken. Many decks can 1-2 HKO a 140hp pokemon, and it also serves as the weakness as a non-evolved pokemon (strangely, I've began teching Mightyena in my deck now. It's awesome with rogue decks and AMU). Mewtwo Lv.X is powerful, but not broken.