What is an 'archer'?

[vfb]

Another difference between enemy ranged attacks (archer fire) and enemy magic attacks is that the magic attacks retarget each combat round.

Once archers target an enemy combat group, they usually maintain that target until it is destroyed or has fled the battlefield.

So you can put a group of decoy shielded units up front, and enemy archers will continue to target them, even if your decoy does something like retreat to the rear (using Guard Commander). Sometimes the archers will miss a round of fire because they are trying to get back into range to shoot at your decoy group.

[JimMorrison]

I'm pretty sure that there is no fancy targeting algorithm.

Fire Closest will target the closest, of course. Any other targeting selection will select an entirely random squad of the specified type. It tends to be harder to see than with melee orders, but they both function in the same way.