I love setting up very large battles between multiple armies in my campaigns. However, I find that in field battles it is very difficult to give orders to my AI reinforcements, and sometimes they just enter the battlefield but don't participate at all.

Ideally I'd like to wait for them to enter the battle, then set them to Defensive so that they will form up properly, and finally set them to Aggressive so that the 3-4 armies can charge towards the enemy lines in a somewhat cohesive mannerism. What usually happens however is that they will enter the battlefield, wait 5 or so minutes, and then send a couple units straight into the enemy, and then follow up with a very disorganized charge. Sometimes they stop short and stand just outside the battle itself. Here is a battle that could have gone a lot better had my reinforcements did what I wanted them to do:
Spoiler Alert, click show to read: 


Does anybody know how to fix this? Thanks