well, if you really want a cheap way to kill him use one of the instant kills. I stumbled on this accidentaly when I came upon the shadow dragon which proceeded to chunk my entire party leaving only my pc mage, who in desperation cast disintegrate at the last second and dusted him
However, while this kills off most of the gear, an improved method I figured out if you don't have a very high level party for Firkraag, cast magic resistance, or a couple of lower resistances, and all you need is a mage around 13th level I think. Just stock up a bunch of memorized chromatic orbs and fire them off, maybe in a ssequencer. Kills 'im dead if you are lucky, try again if you have to. Of course, it just doesn't feel right, or rewarding to kill him this way though, but if your in a pinch...hey, do what you gotta do!