Definitely stop everything for a sec when changing phases and quickly taunt the boss to the tank. Maybe try to use Earth Elemental, he is very good against the boss elemental. Move out of puddles if need be and prioritize on keeping your toons alive. The boss goes down very fast but don't try to squeeze in too much dps on the cost of healing/moving out of puddles. Getting just one cast off every once in a while is enough as long as you don't die.