Help how do I kill Gauth ?

Who is Gauth? It's been a while since I completed that quest. Is he one of the orbs or one of the evil priests?

Gauths are small beholder type creatures. Spell reflection is the easiest, but if you haven't got that spell use summoned creatures to take the damage whilst you attack h-to-h. If you have the collector's CD you vcan buy the Shield of Balduran which reflects there attacks. Later on you can find a cloak of mirrors which reflects all damaging spells and makes beholders etc a walkover.