I had a lot of problems with Tsorig, but I found an easy way to deal with him. Go down to his location and lure him back from where you came and the head up the stairs towards the bridge above the lava pit. Lure him towards the knight across the bridge, and trigger the knight to attack. Tsorig will then attack the knight when he gets close to him (at least he did in my playthrough), and the knight will fight back. In my case, the knight even knocked Tsorig off the bridge to his death (then I just had to return to the bonfire and return to Tsorig's location and pick up his items). Even if the knight doesn't kill Tsorig for you, I'm sure he can be helpful by chipping away some of Tsorig's HP. Sorry if other people have already commented on this, I just wanted to share the tip.