Oww yeahh these god damned rusty skull is trouble. The biggest trouble is, he casts lots of protection spells (which you can't remove) in 2rd round. So in 1st round you have to cause MASSIVE damage and he becomes to demi lich. It's more harder now you can hit him

But there's a way..
Ring of Rams cause 5-30 damage. So If you're luck you can cause 50 or 60 damage it may badly injure our old lich. And then cast "protection from undead" to Nali and get near the skull and stand near him, dont let him pass. Kick him with staff of magi. Also Melf Minute meteor hurts him too. Also Aerie can support with sling +5 (cast protection from undead those!) also spell immunity:abjuration for your these characters, you'll hit him 2 or 3 damage every round, take a walk go outside then return your home you'll see a dead lich there!
Hint: If you pickpocket him before he attacks you, you GAIN 2 ring of gaaxs!!
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"Not for your eyes only, also for your beautiful ears.."
Yngwie J. Amadeus Zoltan "Diamond Fighter of Aerie Lovers Sacred Band"