My first time, he kept killing me, and I even figured out how to kill him. He'd always get that one last blow that would kill me. In the end, I went back to Skyloft, bought a Guardian Potion and a couple Heart Potions, and upgraded them all completely. Only used the Guardian Potion+. Without that, I found Demise to be very difficult and, according people who've defeated him without potions, almost unbeatable.