Ok, I won't ask for too much.
Just for the unit in the center square to turn and change their facing when getting shot from the back. Then at least they'd get the shield bonus and that'd make them last a bit longer when getting shot at. It just doesn't make any sense that a unit would willingly turn its back to the enemy and stoically endure getting shot in the back for fun.

If CA has more time they can also code the case where if 2 units are going to shoot at them from different directions, they should back off quickly to a corner of the square if by doing so they can avoid short range missiles like javelins and then readjust their facing.
If the short range skirmishers enter the square to get into range then as usual a charge is in order.

Or (if they have extra coding time) they could simply just get the AI to make a charge anyhow to the edge of the square when getting shot at, which would necessitate a temporary retreat by the ranged units. You could then engage with your infantry and other troops but that'd defeat the purpose of shooting them first to whittle them down.
If the ranged units don't retreat then continue the charge and engage. If the ranged units do retreat, either stay in place at the edge or hurry back to the center and readjust facing.

I am not asking for the AI to run circles around your troops and trying to delay the victory until time runs out (for non-siege battles). As time is tight in certain battles, that's not really very fair.

Anyway, I think giving some suggestions on how exactly the AI should behave in a particular situation is fair. We can have a decent discussion on merits for each tactic and who knows if CA finds our alternative AI scripts better/less exploitable they might even decide to code them (if possible).