If you gather a lot of zombies and skeletons, and many vampire lords, a few bone dragons, and death knights, you can be successful. It's advisable not to destroy fairies so that vampires can regenerate if they take a lot of damage. You can also destroy them with magic; each elemental can be hit twice with only one type of magic, as they are well-protected against radiant magic. For example: An electric elemental can be easily damaged with an ice bolt.:smile
If you are playing on the highest difficulty, the more gold and resources on the map, the better.