Dominions 3: The Awakening/Special Tactics

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.

Super-combatants
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 be devastating to unprepared enemy armies.