It's really hard to answer this since I predominantly play JRPG's, which are more known for being a cinematic experience and less for being particularly difficult, but I had a horse of a time Chester on inferno difficulty on Ys 3: The Oath in Felghana. The first half of the fight is easy enough, but then it gets ridiculous (but this game is still a blast!)
It's hard because that health bar is all you get. No healing at all. The video isn't mine, but you get the idea (and it is in nightmare, which is the difficulty you have to beat to unlock inferno).