As for the fight itself. One of the Goristros rolled a 1 on its save vs Banish. One of the Goristros got hammered by the entire party, while the other two were held up by the NPCs.
Okay, I see two possible issues here:
1.) Banishment is a concentration spell. If the wizard loses concentration, the Goristro comes back. The enemies should be smart enough try to break concentration, either directly via Goristro charge (80-odd points of damage will destroy any concentration spell) or via Lightning Bolt/Dispel Magic/etc. from the enemy cleric or wizard.
BTW, I'm AFB, but don't Goristros have Magic Resistance? Did the Goristro roll double 1s then or do I misremember?
2.) If there are multiple NPCs involved
in addition to the six PCs, then that party is not just large, it's huge. I believe a Goristro is Huge though, which means that Medium-sized creatures like NPCs
cannot block its movement. It could have and probably should have just charged right over the NPCs to Gore the Banishment wizard for 80-ish points of damage.
Oh, and a couple of tactical observations too:
1.) The enemy cleric wasted time healing the Balor, but in-combat healing is extremely inefficient. It would be more effective for him to Banish two or three of the PCs right back with Banishment V or VI (whatever he can manage). Although Hold Person VI would probably be better.
2.) The wizard can Counterspell the PCs' spells, and he can Wall of Force to cut some PCs off from the rest so that they have to fight the bad guys all alone. It sounds like you tried to cast disabling spells like Hypnotic Pattern, which was a good plan, but I guess the PCs have a Paladin with Bless or something because you say they saved against everything. That's how it goes, I guess, but Wall of Force at least does not rely on saving throws. (Neither does Wall of Stone actually, although it's only castable on stony ground.) Not to second guess you or anything, but if the PCs all save vs. Hypnotic Pattern successfully my next move would probably be to try Wall of Force so we can divide and conquer.
There's no question in my mind that the Goristros + Balor + spellcasters could have pwned that party.