I like the idea. I dont see a "nuke" working though. I'd say for fire, the meteor should effect 9 tiles similar to a nuke, but only attack using the CRUSH mechanic from dwarven druids. And instead of fallout use craters with same basic usefullness, aka takes 3 movement and has 0 production use.
I like the Dragon idea for body, if its a lesser Dragon at 10 strength, with the fireballs and roar. (maybe 14 strength?)
I like the flying
I like the stone skin
I think Death could be changed to make a call for each unit (30% add undead promo, 30% add demon promo, 30% add to stack an equal strength infernal UU, under your control, which is not in
the call list)
I really, REALLY like the deep mist for water, but honestly its too weak as a 9 square cloud. Better make it a 21 square cloud at least

- also movement should be hindered by enemies in this cloud.
and yes, the appropriate tower must be NECESSARY ... but I'd say the creation of the unit should not be a particular level, but the sacrifice of 4 archmages with the tier 3 spells of that particular element. (or rather the 4 archmagi LOSE their promotions in water, and one of them becomes an Arcane Water spirit)