I think bluerepper's experience will only come in custom battles when you didn't set the opponent to attacker and you to defender...

In campaign battles this never happens since the only time you can use this is on defensive, and this is when the enemy armies charge at you. So they won't just sit and wait. I once had 40 Roman units charging me (20 senate, 20 julii) but to their misfortune I was playing Germania, this was my grand army they were fighting, and my grand army had 13 spear warbands and 7 general/barb cav. So. The results speak for themselves--all 40 units massacred (not a single unit made it off the battlefield, believe it or not) and only 50 dead on my side. Whoohoo. It's vaguely therapeutic to see flags turning white en masse.