I agree that the spell of mastery needs to be made more interesting / challenging.
However I have a couple of alternative suggestions.
1) Do it galactic civ style, and have it exist at the end of a long chain of techs. I would argue that these techs should include mastery of each individual element, and some appropriate way of using that in game.
2) In order to cast it someone under your control must be an archmage of all of the elemental magics, and they all need to contribute to the spell.(1 person could be archmage of air, another of fire and water, and a third of earth...that sort of thing).
3) The casting of the spell could include having to go to a shrine of each of the elements and cast a ritual there first before you cast the global spell. If any of the ritual sites is destroyed you would need to restart.
4) The buildings needed to trigger it can only be built in level 4 cities of the appropriate type: tower of the witch in conclave, tower of the sword in fortress, etc.
I feel like a combination of some of these (not necessarily all) would add to the challenge factor enough to make it more interesting.