"Fire Elementals are beings of living plasma, creatures from the elemental plane who have traveled to Ardania for unknown purposes. They often guard Holy Grounds." - From the collected works of His Majesty's Advisor
Elite magic caster
Body of Fire: Resistance: +90 Melee, +90 Missile, +25 Elemental Magic, Immune to Death Magic.
These monstrosities will make minced meat out of your early troops and underleveled heroes. Especially if you're playing undead ("immune to Death Magic"). Very high resistances will make it difficult to destroy them while not losing a lot of units. Swarm them with 3-4 attacks per turn and heal your forces.
Magic users and healers will make short work of them.