It may be that he has resistance to certain kinds of magic. I fought him and died several times before I went and bought some different items. I shot him with Ice Arrows, Acid Arrows, And Fire Arrows. If my memory serves me correctly the Ice and Acid arrows didnt have much effect but the fire arrows really hit him hard.
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Try this before you talk to Sarevok
buy the Wand of Fire and lots of fire arrows one bundle of detonation arrows. use two archers. equip one of them with one detonation arrow the rest with fire arrows. equip the other archer with a full compliment of fire arrows. have your mage or cleric cast animate dead spell place the skeletons between your party and where Sarevok is. equip the mage/cleric with the Wand of Fire. use the archer that has the detonation arrow and slowly walk towards Sarevok when you hear him start talking PAUSE shoot the detonation arrow then UNPAUSE answer the questions as you see fit then PAUSE return the first archer to where the rest of the party is and shot Sarevok with the Wand of Fire repeatedly. and the other archers fire arrows. the skeletons will attack and keep him and the ogre busy while the archers continue to fire at him. If you do it right you will beat him before your warriors get into the action. I beat him and didnt get hit once doing it this way.