"There's a limit to how many we can eliminate with ranged attacks."
Says the one who did overkill on a small group of enemies. Why not use something that's more cost-effective with mana cost per kill, like an explosive arrow, or a multi-shot arrow, or an arrow with poison gas (that's a bit of a stretch), or normal arrows shot consecutively at a pace nearly as fast as a minigun?
Is there no long-ranged artillery magic that can directly target their formation at the back to lower the enemies' morale? Kill or incapacitate the glass-cannon types? Maybe target the supply line (if it exists)?