You could wait until you have better equipment. I had problems here the first time I got to Ars Magicana, so re-loaded. Then I forgot about this quest and didn't return until I was much stronger.
One hit kills are more fun when you are doing the hitting. Try entering the room first, pause the game after he sees you and turns hostile, then use a shadow potion (may need to move after that, if he keeps heading for the last place he saw you).
