[Gandalf Parker]

The AI TRIES to be thinking intelligent.
You are basically making suggestions to the mage or commander. But out on the battlefield the unit will read the situation for themselves. Sometimes all they can see is undead which might change their mind about what to do. Sometimes terrain can affect decisions or even temperature.

Placing archers or people told to attack "rearmost" enemies can be helped by placing them far off to the sides of the battlefield (the top or bottom of the placement screen). I like to put archers at the top (left flank during the battle) set to fire at archers, and put mounted or other 4-footed attackers such as tramplers off to the bottom (the right flank during a battle) set for "Hold and attack Rearmost". That seems to match up well with how the enemy AIs like to place their units.

Also check the range on the spells you are asking for. Sometimes you might want it to attack for one round and then cast if they are short-range spells.

I know this can be irritating but it also helps. If a unit is damaged and incapable of carrying out its original orders it will at least do something instead of just standing there stupidly trying to carry out its commands.

By the way, each commander can lead 5 squads. Use those. Even if you are just breaking up one large group into 5 smaller ones but leaving them all standing in the same spot with the same orders. It can help you avoid things like having them all rout at the same time, or having them all make the same bad decision.