Teach me how to use mages!

[Saxon]

Everyone here has been talking about the straighforward, blow them away, kind of magic. Also experiment with some of the other spells. Body Ethereal was mentioned, a very nice spell. Also look at Destruction (in the demo) which turns enemy armour to mush. Protection has a long range and can help, though it is not as powerful. Strenght of Giants can also help your forces hit a lot harder, while Fear can make your enemy run. The power of this type of spell is not so obvious, but if you are suffering less causulties and scattering your enemy, you will end up on top after a few turns.

[Shovah32]

Weapons of sharpness is a very nasty buff spell, its not too easy to cast but making all your troops weapons AP (within the rather large AoE of course) is well worth getting a mage up there. Casting it on a group of knights (they have deadly lances, broadswords and hooves, although i cant imagine a horse with a sharpened hoof) they can just charge in and slaughter anything. Also if you can cast it you can also protect them with legions of steel.

[Nerfix]

Weapons of Sharpness + Strenght of Giants + Flails =