When it comes to reinforcements as the attacking force, you can change the place were the reinforcment are coming form by setting the reinforcement flag very close to an edge of the map. Doesn't always work though.
You can also replace units quicker by move them very close to the edge of the map and rout them. AFAIK withdrawing units always make them move towards your back of the map.

The taking out units as they show up technique has an anoying tendency to exhaust your troops.