[Redeyes]

Whenever I am on the defensive I send a bunch of mages scripted to slay thugs to my frontline provinces. That's usually swarm, summon lesser air elemental, soul slay, frozen heart, and similar. You don't need a lot of mages for a successful deterrent.

I guess that mostly applies to multiplayer. In singleplayer you should send mages to act as artillery or ones to improve your provincial defense forces with defensive spells.