[Peter Ebbesen]

Too many options to list, it all depends on your capabilities. A few possibilites you might consider:

Thunder ward

Fliers with attack rear

Howl in battle in forest provinces. Unless the mages are guarded, they may end up as wolf-food

Assassinate the mages before the battle

Cast nasty spells that affect the entire battlefield (too many to list, choose what fits your mages)

High-level ritual spells that hit most units in a province. Seeking arrows, fires from afar, leprosy &etc

Commander's with bows of war and orders to "fire rear" or "fire archers" depending on the enemy's troop position


...And a special, since you are Ulm. One or more Golems with faithful, charcoal shield, stymphalian wings, horror helmet, amulet of anti-magic, ring of tamed lightning with orders "body ethereal, astral shield, attack rear". They can REALLY ruin your enemy's day. (Only the Golem and the Stymphalian Wings are obligatory - the remaining items just make the Golem much nastier and more likely to survive trampling the rear echelon of the enemy army)