It would help to post your gear, but...
Balors are vulnerable to humanoid and daemonoid slaying weapons and ammo. If you have slaying missiles (preferred) or weapon (rune-covered trident works well), you should be able to kill the balors with minimal difficulty. If you have controlled teleport (you should), then go to both levers, close them to block corruption, then move away until the balor reopens the gate. Kill him, close gate again, and repeat.
If you have rain of sorrows, that will make short work of the balors as well. If you're desperate, you could try a scroll of familiar summoning if you have one. Use prayers to restore PP if you get low (you hopefully have lots of piety). Or just cast from HP--it's not like stat drains matter at this point. Don't w5 to restore HP/PP. Use potions or prayers. If you absolutely have no other way, HPcast healing spells.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.