Step one: Accept the Scroll from the Helm Ghost.
80,000 Xp per person.
Step two: Kick Demogorgan's ass.
100,000 Xp per person.
Step three: Go outside and read the scroll.
20,000 Xp per person.
Grand total, 200,000 Xp per person, and that's excluding Demogorgan's minions.
Easy way to kill Demogorgan:
Set seven Spike Traps and surround his spawning area with Aerial Servants. It's a instant kill when he spawns. Cheap I know, but if you want a quick Xp boost, there you go.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart