Summons

[Duncan_Frost]

After taking a quick look on the wiki...

It has low HP and default defence so a swarm of chaff, a few high-powered attacks or a good damage spell will kill it quick.
It dies without a mage, so you could try targeting them instead (as you mentioned).
It's lifeless, so it should be vulnerable to magic weapons such as Smasher or some spells like Shatter.

[Soyweiser]

To target the mage. Hope the other guy doesn't bring along non-mindless stuff and cast mind burn.

Also living Statues have very low AP. They move snails on the battlefield. Spells that bypass protection can also be useful.

(Mass blootletting spam for example, they have low mr, so the spell hits often. The spell has a chance of killing the support mages. And the damage done to them can only be repaired in a lab).

[krpeters]

Troops with a high attack and damage will eventually do enough damage to kill them. Each statue costs two earth gems, so he's not going to have a lot of them.

And they do move slowly, so they're fairly easy to go around. Have a lead infantry force attack them head on, and have a cavalry force off to the side with orders set to hold and attack rear/archers. With luck, the cavalry will go to the rear, kill the mage, and that will be the end of the statues.