With melee oriented characters what I have found to work is a slower but safer method:
Run at the boss to bait his side sweep move. While he is locked in the animation maneuver around and hit his ankles. Depending on your weapon you can get off 2-3 hits before you need to move to avoid his stomp move. Stay patient and careful and keep chipping away at him and eventually he falls.
For mages in particular all you need to do is summon Mild-Mannered Pate right before the boss and let him tank it while you spam soul arrow and great soul arrow. If you are not human then run at the boys to bait his sweep move and pepper him with spells. Stay mobile and cast when the oppertunity presents itself.
Basically in general be more cautious and don't run up and spam attack on it and expect to live. Your first priority is not getting hit. Attacking is secondary. Eventually you will get him.