I was struggling with the final boss fight for about 20 minutes when I decided to kind of cheat the game a little (not really cheating but it pretty much ensures you can do any of the game without taking damage...sort of).
I used the last weapon you unlock in the game. I don't know what it's called but it's the one that you pick by pressing down on the D-Pad. I didn't have any magic for the fight so all I had was my weapon.
I dodged his fire attacks at the start then rushed him and mashed the X button. I hit him 3 or 4 times then rolled away. Whenever I saw his sword light up, I rolled away because he was going to attack. He never did his long range attacks on me because I was always close to him. Occasionally he would jump in the air and slam down on the ground. If he did that, I would roll as far away from him as possible.
Here's the trick and it's a pretty obvious one. You can save whenever you want, and if you take damage reload your recent save. This is very helpful on higher difficulties as you can pretty much run through the game with full health. Every time the boss tried to attack me and I successfully evaded his attack without taking damage I would save. If I took damage, I would reload my last save which had me at 100% all the way through the fight. Don't worry too much about loading times as it's almost instant.