I like this idea, units being trained to use different tactics could easily make or break an army.  It would definitely give a new appeal to the Trained units because right now its just a bonus HP adjustment.  Historically, soldiers trained to use varying complex formations and tactics had a distinct advantage over others who did not.  Just look at the Spartan Phalanx, or the Roman Testudo.

However, it doesn't take much 'training' to be able to raise a shield over your head to block arrows; it might to do that in formation, like in a shield wall, but its the size of your shield that makes the most difference, and so how much it covers.  Now staying calm under fire and keeping formation and keeping your shield steady and level in its place would be the real challenge, as you have to remember, soldiers are only human.  Except in games, of course, but then we have simulated morale.

Suggestion: Hero's and sovereigns know specific battle tactics (i.e. traits)

A unit in an army can use any battle tactic known by its highest level hero/sovereign.

Example: Hero Marcus the warrior has the Raise Shields battle tactic trait. Any unit of Experience level x; or greater, under his command can use this tactic.

An unit commanded by a hero that does have this trait cannot use this battle tactic because it's commander is not wise enough to give the order to his troops.

A warrior hero can learn a new battle tactic trait every level while a scholar hero would learn 1 every 5 levels.

Example: 5th level hero knows 5 battle tactic traits. 5th level scholar know 1 battle tactic trait. In battle an army commanded by a warrior will have access to more battle tactics than an army commanded by a scholar (sage).

Allow players to select the commander of an Army. Selecting commander 1 gives you access to tactics A, B, C. Selecting commander 2 gives your army access to tactics B,E,G. Which commander you choose can affect the battle.