I personally thought this was the hardest boss in the game.
It's been a while, but:
I used an Axe, because, as others said, it's easy mode. I'm pretty sure I also used leech on the axe (because it's double easy mode), but I may have changed it to fire for this boss. If I remember correctly, this guy is really weak to fire.
You can summon up to two NPCs to help with the fight if you've done all the quests correctly. I had broken some of the quests in my game by this point, so I only had one companion - the woman.
Again, if I remember correctly, the fight was basically hit-and-run tactics while the NPC tanked the boss. Just get as many hits in as you possibly can, smacking him in the face with the axe.
This fight has two stages, and I found the second stage to be easier than the first.
Make sure whatever armor you're wearing is resistant to whatever the boss is throwing out, and also make sure you eat some stamina food.