I found a thread yesterday in which some people were defending the x-pack's spawning reinforcements all over the map. Shogun is a great game, ain't saying otherwise, but you shouldn't defend bugs.

Anyway, I had a big 2k v 2k defensive fight, and finally hit on a way to slaughter well over half the enemy. After your first route, march your whole army to the point of departure of the routed units, turn around and march just far enough down to get your men facing back to the map, but be sure to be real close to the edge. The new units seem to form up somewhere in the middle, come running at you and then start routing. The depleted fragments make easy pickings next season.

Of course, this does make for an incredibly crappy gaming experience, and is utterly unrealistic.