No, it's a Protection from Magic scroll you need. You can buy them at the Adventurers Mart in the Waukeen Promenade. They have a green symbol, with a moonscythe and three stars in a circle.
My brother got him with his fighter, using the mace he found in Bodhi's Lair (the one that can destroy undead). For some reason, for me he kept saying "My weapon had no effect". Anyway, I equipped Minsc with a WarBlade +4 (secret stash at the Adventurer's Mart), and was able to kill him with that.
It appears you can only kill the Demi-Lich using +4 or better weapons. He is immune to spells like Sunray.
What I did yesterday, was this:
I cast Chant, Haste and Protection from Evil on my entire party. Also I used enrage with my strongest Fighter.
At first, a 'weaker' (that's RELATIVE!) Lich appeared. My strongest fighter kept back, while my fighter/thief attacked him. This is because this Lich is likely to dispell magical effects. With my Wizard I cast a Ruby Ray of Reversal on this Lich to remove any protections it had cast on itself. Once the Lich cast his 'dispell effects', the Fighter took the place of my Thief.
Then, when he was defeated and the Demi-Lich emerged, I used a Protection from Magic scroll with my fighter, and tried attacking him (fruitlessly as it turned out) with the above mentioned mace. With Minsc I first took care of the Mislead-image of the Lich, and next attacked the Lich himself, while the Lich focussed all his attacks on my fighter, but because of the protection of the scroll, they didn't do anything. He went down without too many problems.
A great boon here is 'The Ring of the Ram'. This ring can do damage to the Lich, so make use of it! Your weapon-based attacks will at most do 4 damage per hit, so adding the damage potential of the ring reduces the time required to get him considerably.