just manage your health and hit him with little spells and poisons, and if he has chiv or a casting spell and goes to cast something, go with your explosion and hold it and gain some space, drop it, and cast whatever. The point is to not let your health drop below 60% or so until you know that you will be able to finish them if they stay on the screen. The trick here is to let them get you close to death so they will get greedy for the kill, then you finish them. It might take 10+ tries to pull it off, but it will eventually work whenever they have a missing streak.