In KH1, the only non-optional boss I had trouble with was the 2nd Riku fight in Hollow Bastion. I would try to heal after he hit me with his dark aura attack, but he would throw his weapon at me and kill me before I had the chance. It took me a few attempts to finally win. However, since I could not skip by the cutscenes back then, I had to watch the previous video over an over. I have it practically memorized.