Vampires. Discussion.

Eliminate them using elven archers and ents.
P.S. Naturally, use archers with other races.
P.S.S. I'm not sure about all races, but this scheme definitely works for the Elves.

Vampires can reach the elven archers before the ents reach the vampires. Then both the elven archers and the ents are doomed, especially if you cast a mass plague on all three, vampires are immune to it, and the higher initiative of the elven archers will play a cruel trick on them.