How many 5 earth mages does your enemy have? Chances are not many.
Taking this into acount what can you do about them?
Get a fire mage to put up a dome of flaming death on an area which is often attacked (make a target out of it) a couple of earth attacks come in and the mages get fried. Your aponent decides they're better off using their mages to cast earthquake.
If you scout around a bit you can find the locations of these earth mages now spam seaking arrow/ mind hunt depending on your enemy and hey they don't have earth mages anymore.
Or you can just gear up your mages like you said. Or get a few large body guards that wont be trampled. Hey look a niche use for the iron dragon
!
You can also recruit loads of week comanders as alternative targets. Killing onme indy comander is not a good way to use a 5E mage.
All in all I'd call earth attacks more of an annoyance than game breaking. And I bet astral corruption would really annoy you
Oh and experiment with SCs. When you get something that eats earth attacks for appertizers before chomping through a good sized, mage supported army, and that can teleport, come back and say that about defensive strategies. They of course have their own counters.