Maxi, do yourself a favor. When you see the cut scene with the castle, pretend you won and turn of the game as fast as you can.
I used to do that. I could get all the way to the next to last boss, but after he killed me, the game, in one of the most heinous examples of Nintendo hard sadism the world has ever seen, sent me back to the beginning of the act. This made it practically impossible to practice fighting the boss to figure out how to beat him. I didn't beat the game until college when I realized there were websites that listed Game Genie codes. I played it again recently using an emulator and save states to negate the problem entirely.
(Actually, I screwed up with the save states. When I finally beat that boss, I lost my last hit point at the same time I killed him, but I didn't die. I overwrote my save, then immediately died in the next fight. I had forgotten that your hit points carry over from one fight to the next, unlike your sub-weapon. I had to replay the whole act again, but funnily enough, when you get to the end, you don't have to fight any bosses you already beat, so I went into the final boss fight with the almighty Jump Slash. It was awesome.)