I am trying to kill the demon so I can get the gem. Any helpful hints on how to kill the nasty beastie?

Thanks

Are you talking about the Underdark? If so, yes this demon is a tough one. I used a lot of pierce magic, lower resistance, etc. on him. Then blast him with your mages as many offensive spells you got on ya. You need +3 weapons for your fighters to work on him as well. Anything less doesn't work on him. And he casts some kind of wierd fear magic. I cast remove fear on my parties, and when I ran out of those spells, I just let the party members meander around. They usually stayed away from the demon so stayed safe. I finally won the battle, but two people in my party croaked. At the end of my battle, I had only two people (my main character and Minsc)left with the others dead or wandering around in fear. It was so close! Minsc was already pretty damaged and I was out of any healing spells or potions. But in the very end, Minsc made "Minsc meat" out of him!
Basically, I would suggest anything that resists fear will be very helpful in this battle. Hope this helps.

I guess that I've been lucky with the Balor battle. I've never had much trouble taking it out.

My current party was all set to wipe it out and that damned Balor stunned everyone except my PC (Alarra the inquisitor) and mazzy. Alarra arrogantly turned her back on the Balor and prompty UNstunned the rest of my party with that WONDERFUL inquisitor dispel magic and then we proceded to quickly smash that demon back to hell. (Valygar with the Celestial Fury returned the favor by quickly stunning the Balor!!!)

I didn't really even have much in the way of protective magic up. Perhaps a Bless on the party and stoneskins on Imoen and Nalia.