Teleportation (over a limited distance), retaliation, and a blinding ability (although I might be confusing them with Radiants; they are subconsciously perceived as almost the same unit), and various vengeful shields (when they are attacked, the attacker receives a portion of the damage back + when they die, all enemies around them take additional damage). When fighting against them, it's hard to imagine a worse opponent (except perhaps Phoenixes), as it's quite difficult to resurrect everyone they've killed.
Hmm, I think this is exactly about Radiants and their upgraded version. I wonder what abilities the Light Elementals, summoned by mages of level 15+, have.
And Earth magic is great for healing: HoT on the tank allows it to take damage for several turns while maintaining full strength. And mass healing does the same – but for all troops.
I like Light magic less – if your stack is attacked for several turns in a row, you have to wait for the healing to come off cooldown. Although, maybe a mass shield is a great thing?