I'm not sure what your party/level/equipment/spell situation is but I'll try and give you a few options to cover the ground here.
Now, the Monstrous Manual says that Balor is a +3 or higher to hit monster, but I recall someone saying that he's actually a +4 in BG2. Which is possible since this game really doesn't go straight by the original books.
First, please please tell me that you have Keldorn or at least your own custom Paladin in your group and please tell me that he/she has a Carsomyr...please. If you do, just protect fear on him, and send him in with haste and pray he doesn't get stunned.
If you don't have a Paladin, you're not licked yet. Please say that you at least have someone who can use a two-handed sword well and is NOT of good alignment. If you do, head over to the Kuo-Toa dungeon first and grab the Soul...something...Soul Reaver +4? I dunno just grab it, and use that on Balor. But getting that sword might prove just as tough.
IF the books apply here and Balor is a +3 only, then your task should become much easier, just fear protect one of your guys, give him chaotic command spell or free action and add a dash of haste and send him in.
If somehow you have a Pre-Order bonus disc, you can try the Sling of Evershard, but I don't think that will be able to do it alone.
Balor is one tough cookie so you'll need to drink some healing potions along the way.
Now, if you have NONE of those (shame on you), you can try some spells such as Bolt of Glory, er that other one that hits evil things...the priest one...god, what was it...Holy Smite. The Monstrous Manual also claims that Balors have 70% magic resistance, and through personal experience, that sounds about right. Which means, those spells will need to be accompanied by Lower Resistance if you want to go that route.
Hrmph..Now that I've said it, I don't think Bolt of Glory will work since Balor isn't considered a demon but a planar creature...anyone want to fill me in on this?
If someone becomes feared or stunned somehow, use Dispel Magic, Keldorn is and awesome dispeller since he dispels at twice his actual level.
Man, I just realized how much quoting the Monstrous Manual made me sound like a spaz
One last thought. If you find yourself in a totally dire situation, no weapons, no spells, no nothing, just kill the little bastard gnome and take the gem muahahaha.
[ 09-19-2001: Message edited by: Yuusuke ]