Suicide Casters...

[alexti]

Mistform would help. Bringing more weakly protected troops may help too. I had an opposite problem - my tartarians refused to cast rain of stones, presumably because I had many hawks and cheap undead to absorb enemies first strike. Everybody important was well protected (or had hundred or 2 HP and regeneration, at least). Another possible reason may be that the enemy was not considered strong enough to spend earth gems, but they were spending gems on much weaker armies. I wonder if it's the absolute strength of enemy or relative to your armies is taken into account.