It does not really matter what troops you use and what specific unit you choose to attack, unless your choice is obviously suicidal, such as sending off a unit of peasants against some Spartan Hoplites. Have your men charge the enemy that is in a direct straight line in front of them. Before starting, organizing your troops into a single line formation with Shift+1 so that you are facing the enemies unprotected side. Sometimes the ambush will not work and the enemy will be able to prepare their men before the fight, but if your troops pull it off successfully, you can take as much time as you want to organize while your enemy automatically start out the battle in a vulnerable column formation. Your troops will automatically spring the trap during the enemy's turn when their army passes your hiding one. Otherwise, there will be no ambush and your bait is hopeless.). You can use something as bait to ensure they travel that path, such as a single weak unit that you don't want to involve in the fight, or an important family member with no reinforcements (make sure the enemy will not travel a different path to reach your bait. When the soldier representing your army is crouching but still holding up its sword, you know it is hiding. Do this in the campaign map by placing the units you plan to have take part in the ambush in a forest adjacent to the path you believe your enemy will travel in a next or upcoming turn. If you can pull off a successful ambush, you can destroy an entire army with only a fraction of the men the enemy has.

Rome total war battle tactics