Vampires. Discussion.

that means you lose often......cerberi deal decent damage and if you're lucky it's even better, three at once, and spawn perfectly take out vampires who have the clip wings spell cast on them—and devils are wonderful in both attack and defense!it's cool that they can summon ice elementals!

If we're going down that road: ghosts - age (which is sometimes much more serious than a hit from several opponents), also they have incorporeality, meaning if you fight them manually, it's +100% defense... that's not bad either :)
Everything is already clear about vampires... (I think there will be plenty of defenders here :))
Bone dragons - fear - meaning a hit without a response... (and also I really liked driving units off the towers this way when storming castles)