And I started to think that since the kobolds were so fond of fire, the correct and obvious solution is figure out whether the level's ventilation is, and cut that off. Then, send in some illusions for them to attack. Once they suffocate on their own smoke. . . .
This is of course based on the assumption that this is "rational dungeon" not "weird and magical place" -- dungeons never had air trouble, any more than the monsters had problems eating -- and might even force the party to identify and raid their lair for a magical air source. But it would work.