Killing mages in the back

[Foodstamp]

You can also equip your own commanders with a ranged weapon and aiming items. Then set him to "Fire Rear" or "Fire at Large Monsters" if they are using mages who have high hit points in relation to the other units on the field. I used this tactic one time against a guy playing Kailasa and he publicly called me a cheater . Many of the bow/crossbow weapons work well for this and normally have cool secondary effects. If your commander has access to air and/or nature magic you can increase his precision further with "eagle eye" (I think this is the name of the nature spell.) and "aim".

[Redeyes]

I also think that works pretty well, Foodstamp. You are right on the names

If you can get the proper resistance wrathful skies/heat from hell and other battlefield enchantments are pretty efficient at taking out weak commanders like mages usually are. They aren't immediately effective, but if you set your army up so it holds back in the back row you can easily win a long range war of attrition. Caelum is especially good at this, but other air nations should be able to do it pretty well too.

[Maerlande]

Rain works on fire mages. Fatigue them out and they can't run away.