You could try increasing the casulty heal rate, it doesn't effect the in-battle balance, and means you end up with something left over, with a significant enough boost (although I'm not sure on the limits), you could probably get the recovery rate up to about 30-40% of your casulties, which should leave significant enough remains of a sorely thrashed army, but not make it so the battle was almost pointless (whats the fun in fighting the same army twice, only with 5-10% less men then before?)