How to target specifically with spells?

[Covenant]

I've had a lot of luck using spells just 'in general' with my mages, but I'm having a serious issue trying to figure out a way for my mage to be able to case a spell specifically at one unit or another.

The problem is that the excellent high-damage single-person spells are all but useless when cast on a single soldier, but very effective when they hit a large unit. I want to be able to target large units or rearguard troops like mages, but I'm not sure how to do that. The spellcasting doesn't give me a target the same way attacking or firing does.

Without some more direction, I may end up wasting a lot of potent spells on targets that do not require that kind of firepower. Can someone tell me how the AI decides to target things, and if there's anything I can do to make my spell accuracy higher?

Or at least high enough to single out large units and rear units?