[Kadelake]

Magical weapons are very rare on recruitable troops (except for Caelum, most of their troops have magical weapons). They are quite common on summoned units, though.
If you forge magical weapons and give them to your commanders they (of course) counts as magical weapons.
If you have sacred melee units and a fire 9 bless, that would give them a magical fire attack.
The fire spell flaming arrows makes your archers do bonus fire damage as well as counting as magical weapons.
Maybe the easiest way is using your mages against his pretender? Does he have 100% resistance to lightning/fire/cold? If not, that might be a good way to attack.

The two most common ways to deal with super combatants is to forge magical items and hand it to some good commanders (if you have access to a suitable chassis) or mages.