I'm guessing it's something like Yiazmat from FFXII. Tons of HP that never goes back up even after you break off fighting him, so you just take him down in chunks.

Personally, I like big baddies like this. It's one of the things I loved most about FFXI. As long as it's not 15 hours straight, I don't really see how it's any different than farming 15 hours of worth of regular mobs.