The easiest or quickest are usually if you place your army closest to the enemy line as possible, and have ambushes or flank troops on each wing. When the enemy come towards your centre, envelop them (ambushes add extra morale...um....drop) then kill their general and they scarper. Basically just ticking all the boxes to do with lowering the enemy moral:-

-Ambushes
-Kill General(s)
-Surround
-Superior numbers
-Quick numerious casualties