Fortunately, thats why we have Kangaskhan or Hypno, and those decks have significant weaknesses that Mewtwo can exploit: Charizard EX takes a long time to set up without Moltres (which Mewtwo EX can OHKO) and Blastoise EX requires a lucky Misty draw to reliably get its Energy
But then it was "printed" at Common in a Coldsnap theme deck, even though there was only 1 copy per theme deck, so he was exactly as "rare" as the rares in that deck
As I was not involved in Magic 2015 , I can only guess at the evolution of this design
If creatures you control deal combat damage to multiple players at the same time, Heroes in a Half Shell's last ability will trigger for each of those players
It doesn't matter if Green Goblin was on the battlefield at the time the cards were discarded
I do that too, but it does make me wonder if a deck like this belongs in a more casual meta