Batman would pull out his kryptonite once Superman, Wolverine and a few others take out the Hulk, and most of the other overpowered people/beings involved. Then wolverine (and possibly Iceman) would take out Batman. They would continue to gang up on whoever they perceived to be the most serious threat until only a few remained. Anyone who has an easily exploitable weakness can't win, because someone would come prepared for it. I think that it would likely end in a stalemate between Jean Grey (possessed by the Phoenix) and Swamp Thing.