[Helm himself]

I think i remember reading somewhere that to hit the balor you need +4 weapons or something. Or he might cast prot from magic weapons as Angelousss said. When i did it i just had keldorn with his +5 holy avenger sword (with +5 vs chaotic evil dudes) and minsc (dual wealding with frail of ages +3 and daystar) tank for the party and had valygar and mazzy shooting with bows, my pc (a mage) was casting pirce shield and despel magic. This way i took him down as easy as pie.