Ditto to Kitka's advice. What I often do is summon a LOT of creatures (and I mean a LOT), send them in front of you and inch forward slowly. I managed to get attacked by Sarevok only, without his minions. The trick is to completely surround him with summoned creatures so he can't move - Sarevok may kill 1 kobold per hit, but since I summoned over 30 monsters, that doesn't do much! Then fire Arrows +2 and other fun ammunition at him. At least that's the way I did it. Under NO circumstances attempt to defeat him with tanks, my main character was a tank with AC -10 and almost got killed with the first hit!
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Sapphire Dragoness of ALSB
Waitress at Cloudy's Cafe
Guardian of the Temple of Aerie (specialized in GenCon and BG)
Most Illustrious Arch-Magess of the Illuminati
Lux omnis est