I was stuck on him for hours. I tried all sorts of tactics and finally realized that you have to go Bloodborne on him. I respeced and threw a bunch of points into ADP (I had been using 88 agl), went back and got him once I found a weapon I was comfortable with. My Demon's Greathammer was the one that finally worked, as it has a 360 sweep after a roll, so I was pretty much guaranteed a hit at each opening. It was 5 am and my eyes were throbbing, so I still had a fair amount of slop in the fight, but I stayed calm and lucked out a few times. Sometimes when he uses his homing soulmass-like spell, he immediately lunges afterwords. That got me a few times.