It's only a halfway option, but you could go into the tactical battle and let it be AI controlled (which is a lot faster than doing it entirely manually).  Of course, that might not be a good idea if you have any spellcasting heroes that love to burn mana in your shrill extermination squad.

Actually, the Fire Shrills can target their spell attack (like the Fire spell Fire Bolt or whatever it's called).  The Air Shrills have a spell attack that is like the Air spell that strikes one random unit.  Fire Shrills almost always focus fire in regular combat.  Air Shrills always do damage to a random enemy.  Water Shrills don't have a damage spell, they have the Slow spell.  And Earth Shrills don't have any spells but they have a lot more armor.