Earth Attack

[Cohen]

Unless it's late game and enemy could have tartarians or earth kings or troll king a Dome of Flaming Death should work well in killing or damaging the enemy mage but this won't protect.

All domes are good except the Corrupted one that is pretty strange and from text, damages the units under his coverage.

Probably you can't afford to equip every your guy with items, so or you script some spells or you try to assign bodyguards (useful mostly to give time to your mage to prepare or kill the enemy).

[NTJedi]

One of the best defenses against Earth Attacks is having some cheap priests or patrolling scouts walking with your valuable commanders. Also placing guards with your best commander plus a single wand of incinerate works too.

[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.