If Mewtwo is using X-Ball, your opponent can drop Deoxys, attach for turn, and Colress Machine to it and KO your Mewtwo. You need DCE and Energy Switch to be able to OHKO back. I don't think this means Mewtwo will disappear, though. It is still a good card, and if it doesn't have any energy attached to it, Deoxys 3HKO's it, so it's not a starting liability. Mewtwo players will just have to be super careful if they see Plasma anything.