Assassins
While assassins may be somewhat expensive and their success is not guaranteed, the abrupt elimination of enemy commanders can be highly disruptive to enemy forces.
Army annihilation spells
Armies can be felled en masse. Sometimes this is done with battlefield magic that affect all foes present; there is also ritual magic that can target enemy armies in arbitrary provinces.
Raiding
Not all battles are for conquering and holding territory; it can be worth it, especially for highly mobile or stealthy forces, to target individual provinces for some particular value and do damage before the province is retaken.
Paratroopers
Some rituals like Fairy Trod allow you to move an army in the magic phase into any enemy province. Other spells like Cloud Trapeze allow an single Commander to teleport himself into an enemy province. Other (cheaper) spells like Imprint Souls or Call of the Wild summon a small armys into any enemy province. Combine those tactics with stealth armys already hiding in the target province and attacking this province to suprise your enemys. Force your enemy to patrol his lands and invest his gold into province defense. Keep in mind that battles initated by magic come before normal troop movement. Try to lure strong enemy armys into a bottlenecks and use your magic paratroopers and stealth armys to cut off all neighboring enemy provinces while your conventional army rout the enemy army. If the enemy units are routed and have no province to flee into, all enemy units are lost and your enemy will hopefully think twice before attacking you again.
Super-combatants & Thugs
Particularly formidable commanders, such as Niefel Jarls and a number of pretender types, can become extremely dangerous on the battlefield with appropriate magical gear, orders and perhaps a few spells. Some such commanders can conquer enemy provinces alone and are called Thug or Super Combatants (SC).