You did say
thief, right?
Then you've as good as completed the game already. If you have a haste spell (or a potion of speed) use it, and then make yourself invisible (again either through spell or potion). So now you're hasted, and invisible. Now cast 'draw upon holy might' on yourself. Run up to Sarevok, backstab him with that +3 blade, then run back to the bottom of the temple (where the door is). Use another potion of invisibility (or spell) and repeat untill Sarevok is killed. I seriously did this whole bit in about thirty seconds. It was so easy. Just make sure you have a lot of invisibility spells memorised. Or a lot of potions, of course.
Good luck