My current party has two level 6 fighters, a level 6 paladin, a level 7 conjurer, a level 8 druid and a level 5 fighter/thief. What I do is I first free the captive adventurers from room 6 (which turns the NPCs hostile) and run as fast as I can to room 4/5. In there I cast stinking cloud on the enemy front line as soon as possible so that some of the enemies would get immobilized. I surround my conjurer, druid and thief (who uses a sling) with the fighters and the paladin so that they can protect them while they use ranged attacks/heal the party. The paladin also has a bow which he can use in situations where there arent that many enemies doing melee attacks on the party. I use as much AoE attacks as possible with the conjurer like Fireball, skull trap, Icestorm and burning hands. I hope that the ranged attacks are enough to kill most of the enemies before they can reach my party but in the end I always get overwhelmed by 12+ enemies. The priests who keep on coming from the hallway leading to the room don't help the situation at all. And my AoE attacks damage the captive adventurers who are fighting by my side. I've also got some flaiming oil on my fighters, which helps with the AoE damage dealing. I haven't tried out yet using the entangle spell of the druid to immoblize the second wave of the enemies, but I still wouldn't have enough AoE to kill majority of the enemies before they reach me, and I'd have one curse/cure light wounds spell less to use.
I've tried entering through the room with the healer (3) also, but that means not having the captive adventurers helping out in the big fight.
All the walkthrough I've checked just say "kill the enemies" in a very casual tone like it wasn't a big deal.