Knowledge is power.

You can create a map, place two heroes of a non-native faction (for example, Mages), set their stats to 0-0-20-20 (it's important that attack and defense stats are zero). Give both heroes Divine Power and Skilled Light Magic. Give one the required number of zombies, and the other hellhounds.
In battle, wait until the heroes cast Divine Power and then simply have them attack each other; it doesn't matter who approaches first.