[ano]

Use bog beasts and you will have the minimal causalities. Bog beast is one of the most useful simple summons in the game due to exactly this reason. 6 bog beasts placed properly will kill you up to 40 priests/mages. I use it nearly in every game I play and I never regretted it. Races that have other summons with poison cloud (or hydras) may use them, of course.
Another option is, of course, chill. Should work too but I never tried/tested it.
Anyway you definitely need something that kills living beings and doesn't affect undead. Namely Cold or poison.