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Nathros, Brandon, and Ash will be on the front lines, but they will be to the side when the volley is to occur - so they aren't in the way. These three can still open with ranged attacks if they want.
Next, a row of ranged attackers (anyone that has ranged and isn't already in the front row). Theoretically, that's everyone else. Even Alanna has a sling.
These people should be bracing against nearby trees for partial cover if possible. If Ash needs to explain this concept, she will (briefly) as everyone starts walking/jogging.
Everyone should have a ranged attack or magic for this - I think.