As I see it, the thing about volley firing was that it was to compensate for the low accuracy of the individual guns. These units should not be capable of picking off an enemy general, for instance. What they can do is fire at a solid block of infantry or cavalry and hit something as the bullets pass through the depth of the unit. Loose formation should largely negate this effect but equally diminishes its attack strength, so one only needs to time the volley for the moment it inevitably closes its ranks at the last moment.

Strictly speaking, arqs should receive a progressive penalty according to how small the advancing unit is and how many ranks deep it is. For instance, a 15 man cadre, reduced to a single rank has no depth to it, so the bullets now need to be accurate to have a hope of hitting any of them.

On occasion, when using a pez unit as stationary arrow fodder, as an alternative to loose formation to reduce the casualties, I've tried spreading them out into a very wide formation, just one rank deep. Because arrows scatter forward and back (variation in range of shot) as well as side to side (in crosswinds), it becomes harder for the arhers to precisely drop their arrows onto the single rank and I've had some success with this method. Just get them to run like hell if a unit advances towards them as they can't fight effectively in a single rank. Arqs might have some success against this tactic but you only need to set them in loose formation to make the gaps so big that they keep missing.

Beware that a single rank of 100 men takes a long while to properly space out to loose formation standards or vice versa plus the low morale units will sometimes spontaneously rout when they sense a lack of fellows to front and rear of themselves as individuals. Being so spread out, a hit by a tight cav formation might only score 10 casualties, instead of the 50 you'd expect if they tried to fight back but the remainder will rout and scatter anyway, which is not exactly an effective use of a unit.