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SPOILER!!
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Well there are of course the cheesy tactics, such as:
1) Feeblemind him
2) Lower Resistance, Polymorph Other
3) Cloudkill him while he's still in his non-aggressive mode (so the circle around him is blue)

Other than that, you can also try to beat him the legitimate way; just make sure you are buffed up quite good (Stoneskin is particulary useful, as is Protection from Fire, prevents you from getting massive damage from his Breath Attack). Summon.. A lot! Skeleton Warriors, Aerial Servants, Fire Elementals, Wand of Monster Summoning. Anything will suffice, as long as his attention is kept away from you. Keep hacking at him until you get decent hits (make suret to dispel his Stoneskin using Breach!!)

Magic users must lower his resistance (using the Lower Resistance spell), and then they can use all sorts of spells. Magic Missile is pretty decent, as is Skull Trap. Lightning bolt will also do the trick.

Good luck!