Ok, even planning an attack on a balor at level 11 must involve a LOT of metagaming, or some good knowledge rolls. You might try those, first.
I'm pretty certain you can dish out enough HP damage to kill it in one round, its AC isn't so high, (if you can get around the DR), unless I'm mistaken. However, this requires that probably ALL of you have to be right there next to it in round 1 to surprise-insta-gib it, or else it'll act, which means you'll then be facing two Balors and a host of SLAs and other abilities.