Okay, so anyone who wandered past that spoiler tag probably knows there's an honest to goodness, 100% immortal, god-level demon prince trapped in the bottom of the Keep. From the context, it doesn't even sound like it's an avatar- they're worried that if you kill him, you'll 'merely' banish him to the Abyss. You TRY to do that with avatars. The fact that the supposed ideal solution isn't to kill him, but to re-imprison him, suggests that what you're seeing here is the real deal.

I have the sneaking suspicion that in the very end-game, with ~6 million experience, it's possible to tank-and-spank him until he screams like a little girl, while you mine his demon army for about 200k experience/minute.

The (theoretical) setup:

CHARNAME: A seriously nasty undead hunter-type paladin. Carsomyr is just the thing for dealing with uppity demons who think that stoneskins or other defensive spells will keep them out of trouble.

Key items:
Ring of Gaxx
Ring of Free Action
Amulet of the Seldarine
Big Metal Unit
1x cast Magic Resist (+40%)
2x potion of magic shielding
?x potion of mind focusing (+3 to dex/int until dex hits 25)

80% base magic resist, 100% resistance to all damage-causing spells, immune to poison/disease (Gaxx), mobility restrictions such as hold person (ring/class) and level drain (class). Alternates between The Answerer (-15% magic resist and -2 AC per hit)/Shield of the Order, Carsomyr (dispel magic on hit, +50% magic resist), and whatever looks like fun to whack somebody with.

When all is said and done, he's also sitting on a -25 AC. Can't touch that.

Korgan/Minsc+Anomen: The mobile chainsaw unit. Sure, Anomen isn't that much better than a pure cleric by ToB. He can still pick up a few more proficiencies in the right places, especially if you're using the Level 1 NPCs mod. He's also a good off-tank when rocking those stored-up mind focusing potions with a shield, and a great source of level 7 clerical spells. Get some Improved Haste from the back row, keep those level 5 Righteous Mights up, and you have a quite decent 4 attacks/round with max damage via your favorite blunt beat-down instrument. When bad things happen, he can generally fix them before they get fatal. Minsc can wear Montiolo's cloak, but Korgan's probably a bit better overall. Either way, shove a pointy thing in either hand and point sharp end at enemy.

Sarevok: Why Sarevok? Because once CHARNAME has Demogorgon nice and occupied and completely vulnerable to both magic and physical attacks, Sarevok can walk up with Soul Reaver, pull a Greater Whirlwind, and apply a near-instant +20 THAC0 penalty that lasts twenty rounds.

Unless Demogorgon has some kind of special "But that's not fair, I'm a god!" immunity to weapon effects, he is now going to be impotently flailing his tentacles while Imploding whoever is closest. Remember how you're 80% magic immune, 100% immune to magic damage, and laugh at hold person? Just stand by him and let him think he's going to get lucky or something.

Aerie and Imoen (because she has better variety than Edwin) make up the back row, and can pop Improved Alacrity and focus-fire on any problem spots, or use wands to Breach annoying Maralith protections.

Once the Maraliths are down and Demogorgon is about as scary as a kobold, he'll summon 10 Glabrezus and try to maintain them. With a THAC0 of 11, they're no threat. The -7 AC and 80 HP mean Anomen ALONE will kill 1 per round.

Commence beat-down until you run out of spells.

But that's just my ideal method. Given any party makeup, and any selection of end-game equipment, how would you do it?