Training -

I think it is best while you have the chance before you go on the offensive to train all the riflemen, because then if you lose a few in the process of an attack you don't have to worry about not having enough to continue with your tactics, not only that the riflemen are US and they don't take that long to train.

You could decide to train the jeep(s) then you will have the fastest vehicle in the mission (see tips), also you will find that if they get hit they will repair themselves if the life (green) bar is still around 50%.

If you have the patience you can even train the Calliope which will besides other benefits, reload quicker and be very accurate, also another tip is to replace all the gun crews with trained riflemen because this will make the guns very accurate, then you could train these to replace the ones used.

Make sure that the Calliope's are not to close to the front line otherwise when the attacks start you will not be able to target the enemy tanks etc, to check click on them and then press the D key and if the cursor is the normal red oval shape then you are o.k..

If you get a red X or a red oval with a green X in the middle then you are not within range of that area, the red X means you are to close and the red oval with green X means you are to far away.

Save before doing this in case you provoke an attack....
Clear some of the trees away from in front of the plateau to give a good field of fire, otherwise the enemy will hide behind them and you will not be able to see them, also your units will not have good LOS (Line Of Sight).

Place a couple of trained officers close to some of the guns, something like one on the plateau and one each side, this way you will see the enemy before they can take aim.

Don't let your supply trucks re-supply ammo while the battle rages otherwise you might lose them; wait until there is a lull in the attack.

Tip -
As an experiment try using one of the Howitzers both before you change the gun crew and after, the accuracy difference with the trained crew is amazing; you hit nearly everything you are aiming at first time even through the FOW (Fog Of War).

Don't forget Experience levels, they are so important, because otherwise when you are fired on and you wonder why or how. Because you cannot see anything..........that is because the enemy is more trained than your units and so can see you when you can't see them.