It may be me, but I don't know if a hero in Rearward stance has to declare an engagement at the beginning of a round or if he can choose his target when his turn arrives. Of couse, the second option would lead to the Rearward archer being able to choose the most damaged foe (or whoever he sees fit) after all his companions have attacked.
This are the quotes from the Revised Core that make me doubt (p.174):
From here I'd say that also archers must declare whom are they going to attack this round.Engagement
When every player has determined the stance his hero
will fight in, all combatants must engage one or more
opponents.
But...
A character is engaged when paired in close combat
with at least one opponent.
More Enemies than Heroes
When the opposition outnumbers the company, the
Loremaster first assigns an opponent of his choice to every
companion in a close combat stance (heroes in Rearward
cannot be engaged).
From the extended combat example, I'd say that the archers can choose their target just when it's their turn to act:More Heroes Than Enemies
(or sides equally matched)
...each player chooses an adversary for his
hero in a close combat stance...
When all heroes in a Forward, Open or Defensive stance
have been paired with one opponent...
And the same happens with the enemies:Finally, Trotter acts. He is unengaged and free to attack
at leisure. He chooses to try and finish off the Attercop
that Lifstan hit with a serious blow, hoping to get rid of it
once and for all.
Would you agree that archers are free to fire at will, after seeing what damage their melee companions have brought on the adversaries?As his final action for the round, the Loremaster chooses
what the unengaged Attercop will do.
Even Prepare Shot doesn't need a target declared beforehand.