Nothing has synergy with Mewtwo X. Mewtwo X is a tech... nothing more. Mewtwo X should not be played in a deck. He is a 1-man show basically. You should either play him alone or as a tech. Once he takes up the stage, he will not let go. As a 1manshow, he fails at being very good. He is slow to set up, and AT BEST takes 1 kill every 2 turns. That just isn't good. As far as a tech goes, he is a pretty poor tech. He requires AT LEAST 2 psy energy and barely counters anything. Most SP players will either be at the back tables or will know how to EASILY play around this guy. Even without Mewtwo counters, SP decks play fine around him.
I'm guessing people will say Great Celebi has synergy because it also walls stuff, but it also fails at walling. Celebi does very little damage and cripples to anything that does damage that isn't basic.