Alternatively, the cleric opens a portal to the elemental plane of water and floods the dungeon. The wizard casts a few chain lightnings into the water. Wizard casts mass water breathing. Cleric empties the water at the bottom with a portal to the plane of water, but in the air.

That does take a fair bit of setup to achieve though. You can also use poisonous cloud or incendiary cloud to do similar things. I used incendiary cloud on a rather large colony of psionic spiders. They weren’t particularly large, but that just meant they could make their psionic abilities more powerful by dog piling ontop of each other.