Hmm...

We discovered one thing in a discussion with Echo...


You really don't want to let us have control of a charismatic mage with a head for tactics and interest in war...
He or she would end up providing a small force of soldiers with the means to eradicate entire armies, all without needing any mages of great power. All he or she would need is to collect numerous relatively weak mages who'd serve his/her cause.

Not gonna describe the exact means, but lets just say that defensive magics aimed at defending the mundane soldiers would be quite necessary against such a unit if their opponents seek to keep their soldiers alive.