Equip the Hand of Pyro and Bite of Lightning. Attack and destroy the two arms, then the white-orb. Block all the attacks Gaia does. Use Bite of Lightning to easily destroy the little robots Gaia throws at you. Do not call the Elemental Summon; you will not have enough time. When only the head remains, shoot Hand of Pyros at Gaia until he dies. Remember to block all attacks.