Protection from Magic scrolls can be bought from Ribald, in the Adventurers Mart.
If you have a Bard, cast Spell Immunity( Abjuration) and wail on the Demi-Lich with Mace of Disruption.
Melf's Minute Meteors, also hurt him, dealing 1-2 damage per hit.
Ring of Ram also hurts Demi-Lich and knocks him back too.
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If not a Sorcerer casting total oblivion, then a Paladin meting out justice. Otherwise, I think I will just be a bard and torture my enemies with, "My Heart Will Go On".
Platinium Sorcerer of ALSB,
Aerie
Loving
Sacred
Band! She's MeowRrrr, and more!
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Baldur's Gate Item Mart