OK, this is starting to remind me why I learned to hate D&D3.5 rules with its linear scaling (thank God for Savage Worlds and D&D 5E with its bounded accuracy). [SPOILERS] I simply cannot win the fight against the Balor! With its 300+ hit points, huge AC, and SR 21+, nothing I have can hurt it. My party of 10th-level PCs including a Hill Giant, all of whom are buffed up the wazoo, have been defeated twice in row with the demon taking perhaps 20 points of damage. So angry. Not sure what idiot designed this encounter (apologies to Gary G if he did so), but it is annoying. At this point, I'd be happy to cheat code the damn Balor out of existence. The only thing I can think of is in one encounter, I didn't have Lesser Globe running - but it does not matter as the Balor uses Charm Monster on my giant and that's it. Advice, help, I want to finish this game!
Oh FFS. Doing a search, looks like simple Protection from Evil is key. Wow, slipping in my old age Does Circle of Prot Evil still work or is it still bored?
Protection from Evil is needed but beware the Balor's fear, having a sword of answering from the Prince is VERY helpful
PCs with good stealth can often walk right up to all sorts of enemies and attack with surprise. Add an improved invisibility on a rogue for sneak attacks. Also, holy weapons are very useful.
Potion of Greater Heroism, or a Paladin's aura is useful against the fear effect. Or a Remove Fear spell. Dwarven Defenders don't run away. Mr Gygax might have put the Balor in (I'm not sure who wrote the original modules) but it won't have been in a the (roughly speaking) 3.5Ed version that is in this computer game (though some of the changes to 3.5Ed definitely benefit the player).
If you have New Content installed, you can finish the temple then fight him later. Properly buffed and prepared, you can beat him at level 3 on most party members. (I don't recommend it though.)