It is a good idea to create a new save before entering new building/areas so you at least have to option to reload if you are run into trouble but can not retreat.
How many potions do you have? As long as you do more damage to him than he does to you, you can trade arrows and drink health/rejuvenation potions.
Do you have any spare skill points? Save and try out different tactics. Summoning a demon should help as a distraction. Rush Attack has a chance of stunning briefly, so you could save, rush attack, if he gets stunned get another hit in, otherwise jump back and switch back to a bow. Poison Arrow or Fire Wall, etc could help.
You can always run back up the ramp to heal, though he will recover his health as well if you wait too long.
