Tiny Critters

I have an upcoming adventure where the magi and their retinue will enter a faerie regio and face tiny faeries that believe they are armies from various periods in classic Roman history. The first army acts as if it's Hannibal's army at Cannae, attempting a double-envelopment on the players; later armies will be Caesar's crossing the Rubicon and late Romans defending against the Visigoths.

Given that these creatures are, at best, Size -3 (and more likely Size -4), how does one resolve combat with an entire phalanx, or a fusillade of tiny arrows? The "group combat" rules in ArM5 make the encounter either incredibly easy but tedious (swarm causes no damage, but attacked individual can only kill one at a time) or incredibly deadly (literally hundreds of critters inflicting at least light wounds at once, all rolls afterwards fail from having at least a -100 to all actions).

Does anyone have any ideas on how to make this a challenge, but not a deathtrap?

I would do group combat but each group is a phalanx/legoin squad so the double envelopement might have 4 or 6 (2-3 per direction) lines of attackers. It would be a trained group with only front 6-7 fighting but back ranks filling in as front ranks die.

THe damage is made up from the extra pluses the group leader gets for the trained group attacking together.

How about each unit being a single faery? So the 16 legions at Cannae are 16 faeries fighting in a trained group. Each faery (=legion) is used like an individual fighter and any damage dealt to it is treated as a would thus reducing its capability to fight just as wounds do to humans. You may describe this like the number of fighters being reduced but can you need not stick to cler numbers. Incapacitating wounds may result in scattering the units while deadly wounds whipe out the entire formation.
Vis of these faeries might be contained in the standard each of them is carrying.

I like this idea.

To further expound, each wound level could equal a certain number of tiny fairies vanquished

I would not do that, since the wound system is not metric, like "the other game". Since you can theoretically sustain an infinite amount of damage before dying, you could be out of tiny faeries before the true faerie dies...