Bhruic and Demon both have made good suggestions - place your archer units 2 ranks deep and shoot at cav, other archers, and infantry without shields. Shooting into the rear or flank of a unit helps.

You can watch your archer unit and see which men in it are firing. If the unit is far away, or your unit is in trees, or in deep formation not all men will fire.

When I read the original post the main thing that I saw was that the archer units were being overtaken. The archers should be placed out in front only so far as they can be protected or can retreat if attacked. If archers are set to skirmish they should be able to stay away from most inf. When the enemy rushes hit Ctrl-M (this selects all missiles) and then Alt- left click in the center behind your infantry. This will cause all of your archers to retreat to behind your lines and become protected.

When an infantry unit is fighting AND taking missile fire the rate of death goes up, which means fewer soldiers and lower morale. Units also lose morale when taking missile fire, so shooting archers at enemy infantry units while they are fighting can cause them to rout,

which is nice

ichi