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. Damage can take the form of, e.g. capturing a province, cranking the tax rate up to 200, optionally hanging around for a turn to pillage or blood-hunt (thus pushing up the unrest even more), and then leaving it with 1 PD, fully expecting the enemy to take it back. (And if not, oh well. You keep getting taxes.) This works particularly against enemy fortresses if you don't have time to take the walls down and thus can't build PD or expect to keep the province long-term.
No Survivors
Routed troops will flee into neighboring friendly provinces. Make sure that no such provinces exist for your enemy ! Use the "magic" movement phase (together with stealth armys) wich comes before the "normal" movement phase to conquer all the enemy provinces around the main "battlefield" province. If the enemy army in this main "battlefield" province is routed and have no province to flee into, all enemy units are lost and your foes will hopefully think twice before attacking you again. Check the army size graphic after such an battle. Take a screenshot from it and send it to your human opponents as a warning.
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. Such magic 'Paratroopers' armys attack together with stealth armys already hiding in the target province and having the order "attack province". 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.
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).