[lch]

I am under the impression that fire/attack anybody always means "the troops" first, not the commanders, and mages don't count as archers, as commanders don't count as "rearmost" unless they have some normal units around them, too. Otherwise you could always win a fight pretty quick by building some flying / cavalry squads set to "attack rearmost" that attack after a few turns, passing by the slower main army force of the enemy. I would expect that "large enemy monsters" might be an exception to that.

Maybe this is not entirely true and the units just calculate their targets by maximum possible damage they can deal, in which case tightly packed infantery are "better" units for attacking to the AI than scattered single commanders.