Game in the Brain

Edit: Check out +Mark Langsdorf in Skirmish Archer Tactics, which provides advice in how DF archers can take full advantage of Heroic Archer and Weapon Master Bow.

Regarding the Pregens
You cannot hope to hit or be effective in GURPS archery even with all the Heroic Archer and Weapon Master Bow. Even when you bring your self to 99.999% chance to hit from 10-15 (-4 to -5) yards (will take mostly 2 seconds to reach you if you were standing still, but your not) you still have a success rate based on your opponents defenses. Regular Mooks have a (Base 3, Shield 6, DB 2) 11-12 thus they have a 60-75% chance to defend with their shields.

as DC points out your damage CP vs damage output is expensive paying 20cp in strength for 1dmg improvement

This is where Horse Archers Come in, with horse archers you don’t shoot just anyone, you shoot who ever turns their back to you… why would they do that. It is because you used your superior mobility to get to the flank while your slower and more melee powerful allies move in from what ever direction they were coming from in the first place.

If you dont have horse archers, stealth and running allow the archer to get to flanking position on time. Since this a maneuver too far from tactical coordination you use Strategy instead even if it is a small force. First of all, you cannot communicate so you need to PLAN and tell the ally a set of parameters that will help best in the situation.  Up to the GM actually, but I find the penalty of -6 tactics to strategy default very very weird because in my Strategy studies, people keep moving back and forth between the two so often in many situations. subject for another post.

At that point, you need to be able to shoot at a range a -4 to -5, from a 17 plus 3 accuracy, you have a 15-14 to hit, and you can still make a called shot to the leg or vitals.

As Vaclav has observed Leg is my favorite target because the options are very generous and the penalty is relatively light -2.

With leg shots

• Chance to knock someone prone – that would put them out of the game for 2 seconds. You can focus on other enemies while this one is out. 
• Easy chance to half their move ability and thus affect their dodge. Like above, but using superior mobility, move combat to an area this character cannot go into. 
• Easy chance to give them bad footing penalty
• plus all the regular Major Wound and Stunning rules. 

One you have movement superiority you can dictate where the fight is going to be by moving away from slower weakened allies, moving to stronger defend able terrain while they follow, or coordinate formations with allies as the enemies react slowly.

As compared to arms hits, they can still be threatening with their shield arm used to bash, knock down, or Aided Defense to their ally.

I am biased to mounted combat because of Mount and Blade, and my own desire to own and ride a pony for both athletic activity and simulations of endurance and military operations.

In Mount and Blade, the range penalties are harsher because opponents can use shield as cover, my shield as cover is a house rule, so I don’t know if other game tables use it as such. When they use shield as cover I target the feet, because they cannot see where I am aiming. On Horse back, its quite a feat in Mount and Blade to shoot the feet since it is a FPS-like game but factoring your own horses velocity and the movement of the opponent using intuition gained from practice.