You do not actually kill Knorr Oaken to win the fight, just get his health down to about 20% and he will declare his defeat. There is time to buff with spells prior to the combat. Additionally, Knorr will only attack if you come close to him, so if you stay where you appear, you have all the time you want to prepare. Anything that will slow him down, as Pin Down will be very useful as well, since he is melee.