[baruk]

I've used earth attacks quite a lot, and I found the toughest targets to be ones that chain-summoned elementals in defence (though this was in Dom 1, when they didn't cost gems).
Summoning spells can be quite expensive in gems, but may be useful for protecting key commanders. A few elementals should be enough on their own to take care of the earth attack, other battlefield summons such as wolves and undead should provide enough distraction to give a few more turns of spell casting.