I, too, disregard the RAW. During the declaration phase the player chooses a number of parries (using the rules in Ars Certo (Guild Companion)), or a total amount with which to parry. When an attack is made against the character, one of the parries (or some amount of the parry total) is used to parry the attack.
Note that this approach makes defending oneself easier. My biggest issue with the RAW is that once you are outnumbered, you need to even the numbers ASAP. So, a fighter facing two opponents needs to drop/stun one opponent immediately because he cannot parry the other one. Thus, I have long played that a character can parry any opponent, not just the one he is attacking. (Although opponents with situational modifiers incur penalties that can be overcome with Reverse Stroke skill.)
Ars Certo summary:
A half parry requires 20% activity, and a shield requires 5% activity (but neither is an action). Each parry reduces the opponent's attack by OB/2 + Parry bonus. Each shield use reduces the opponent's attack by the shield bonus. If you parry the same opponent twice (to get OB + Parry bonus), you cannot make an attack.
In the example you gave, to protect against the archers, each character would need to declare at least one shield use (5% activity). A character can choose to use the shield bonus against a melee attack, but a subsequent missile attack would ignore the shield (unless the wise character declared two shield uses for 10%). Thus, a conservative approach might be to: half parry, shield twice, and attack with the remaining 70%.
Using a more standard parry system, I would allow a character to split his OB into three parts: a shield parry, a weapon parry, and an attack. Although, as others have noted, even that's not RAW.