You can only win with a strong card like this if you use a strong one too hahaha. I find it easier to deal with this deck using either Blaine's Deck (Ninetales and Rapidash) or Starmie EX and Articuno's Deck. Both are very offensive, so either you defeat Mewtwo very quickly or by using Sabrina you can often break the opponent's deck and defeat Gardevoir before reaching the final stage!