In the melée of brutal battle, if Edward had hired the 10 best Welsh/English archers to wait and look for Wallace either at the beginning of battle, or at the end if he insisted upon chasing people off-field, bingo!
Do you realise how difficult it is to aim for a whole crowd of people when you're being attacked and possibly under a hail of arrows yourself? LET ALONE seek out a single individual who is running around, ducking, and being obscured by others in front of him constantly?
If the target was by himself in a field or in the woods with his pants down taking a shit, then yes, you'd stand a better chance, but not right smack bang in the middle of a hectic bloody battlefield.
As ridiculous as this NON-history action movie is, even the producers wouldn't be so stupid as to do this.
Yes, I do realise. Obviously in melee's it would be awkward, but it's not as crazy as it sounds, if Hollywood can do Robin Hood type accuracy, then pre-battle potshots aren't beyond the realm?