How to level minions efficiently (especially spellcasters)

[Aotrs Commander]

I have had little trouble levelling most of the minions, but the spellcasters (and executioners) seem to be problematic.

I am at the point I want to be ranking my spellcasters up to max rank.

I have been trying to send them out on missions, but this doesn't seem to help a lot. I was under the assumption that minions on a mission got "XP" (or whatever), but now I'm wondering if it is only for the matching types? (I.e., I've been sending a caster with a collection of guards/spies on a guard or spy mission. This doesn't seem to have significantly helped?)

Is it just that basically it's a waiting game in the library for them to rank up doing mana creation?

(Which I got just before finishing the campaign.)

[Aotrs Commander]

(Apparently it is partly a waiting game, but yes, sending minions out on missions does rank them up, it's just a fairly long and grindy process.)

[Drusk]

In late game i can sometimes recruit minions at rank 10, so i'm ending up mass recruiting to get rank 10 spellcasters, then disposing of the lower level ones, and repeat