Play as Inferno.

Don't lose any Imps before the final battle, and the Town will be conquered.

Well, I wouldn't be so optimistic. Imps are certainly a big advantage against the Town, but in practice, things aren't always so rosy. First, it's not easy to accumulate Imps, and on level 15, with proper skill development, the Town has a minimum of 7 knowledge (without artifacts), which is 105 mana. Where do you get 400 Imps? Second, it's even simpler: against the Demon, you take Sharp Mind and the first spell will eliminate the Imps. But sometimes the Imps go first, so you need to be prepared for a 50/50 chance. Run to the Inferno with a group of Hunters, and eliminate the horned ones. If it fails, retreat and return again. As soon as the Imps are gone, the Demons will have a hard time. But here, skill is what matters, not the race. And the one who is really the most terrifying for the Dark is the Orc with Chaos Shield.