I can see the Mega Mewtwo w/ Psychic Infinity as a backup attacker w/ Gengar-EX plus Dimension Valley, Mystery Energy + Weakness Policy/Float Stone, and BT Gengar to make a 1 Energy KO happen in 2 turns... Just night attack, switch to BT Gengar(after Rare Candy on Gastly if necessary), attach basic psychic energy to BT Gengar, KO delivered!
^ If BT Gengar can't KO, Mega Mewtwo can clean up w/ only 2 colorless(1 in DV) needed to attack. 1 DCE and a Muscle Band is enough to KO most non-Mega EXes w/ 3 energy already on the non-Mega target. It's like: "beat that, Jack!"(I don't mean to use that language personally, but given that deck possibility is a killer idea, not sure regarding other ways of wording...)