In custom battles my artillery consistantly ignore my orders.

If I tell them to use explosive ammo they usually will, but not always. No matter how many times I try canceling the orders and re-issuing them they insist on using regular ammo.

Even more annoying is trying to get them to target a specific taget. In custom battles if you autoselect an army for the AI it usually (always?) selects 3 units of missile infantry and 3 artillery units. I want my cannon to target the big chunk of heavy infantry deployed between.
I usually start the game with fire at will on.
They taget the archers.
I want to mess up the melee infantry which are so packed together in a big juicy target so I select a target and the artillery continues to target the archers.
So I turn off fire at will and again click on one of the infantry units
either they refuse to fire at anything at all or they continue to fire, at the artillery

"either we fire at the archers my lord or we go on strike"

Can somebody please explain what I should do to get them to fire at the target I would like to.