Ismeant to be fought with help because I've been trying it Solo and getting absolutely destroyed in mere seconds.The Rotten boss
Roll through attacks. If the attack is sweeping from the left (his right), roll into and through the attack. It's really hard to block most of his attacks. The stam drain is brutal on his stuff. Keep in mind he can do his overhead attack up to three times so be careful with that. A dodge to the side will keep you safe from it though. The grab attack is totally unblockable, but really slow.
Edit: So I'm not totally repeating what someone previously said. Try to pull him towards the middle of the room. It gives you more room to dodge around without having to worry about the fire pits.