The key is getting behind him. If you just circle him with your shield up, eventually he'll go for a power attack and miss, leaving him vulnerable(He does a forward lunge where he slams his sword straight in front of him. This is your best opportunity, as it's very easy to avoid if you know it's coming). Just take your time and try to keep him away from the fire pits. I had some trouble with him initially as well, but trust me, once you get a feel for it, he's one of the easiest bosses in the game.