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.