If you are taking a lot of damage, try bringing up your equipment screen and let your character rest for a little while. You can also heal, then quickly press "e" (or whatever key you have designated for your equiment inventory) and take a breather. You will regain some of your health this way, but not the wraith.
Save right
before you go into the room where the wraith is. If you die, you can reload the game and try again. (but don't--whatever you do--don't save INSIDE the room once the door is shut. The door won't open again.)
Just keep plugging away at him. It might take some time, but he will die, and you will know you accomplished it without resorting to cheating...(which is impossible anyway, no cheat codes)

Best of luck!