That Gastly in the anime was also pretty broken, right? I mean, it could summon a mongoose against a snake, a fire extinguisher against a Pokémon that dies if its tail flame goes out, and could fuse two fully-evolved Pokémon together. I'd like to see how well Mewtwo could stand up to that.
It wasn't meant to be usable at all. If I ever printed it, it would probably be as a jumbo card (especially since the text would be so small if printed at regular size). Some jumbo cards have pretty broken effects.