Lower resistance, greater malison, then finger or death or some other instant kill spell if you can cast it. Also cast some summonings to use as cannon fodder and lower his HPs to below 90. Then cast stun on him and keep hammering away. It helps if you can cast neg plane protection so you don't get level drained.
Of course if you're desperate you could also try the cheezy approach and cast cloudkill on him and move out of range.
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