Well,
I'm not the greatest mage user, but I have gotten stomped by several different configurations of mages casting different spells.
One constant I've noticed, people tend to build armies around spells, rather than look for spells that compliment an army.
By this I mean, find a useful/damaging spell (or set of spells), Yvelina gave 2, wrathful skies and horrors, but there are others especially if you play modded games. And then find a country with national mages who can cast those spells with minimal buffing. Then for the army just throw in some 'blocker' type troops to keep the mages upright long enough to do your damage.
I haven't really explored this part of the game thoroughly yet, but I have been on the receiving end of several very effective firing lines
.
Crash