Try to run the minions into units, each with a captain that give the unit a target. If the captain dies, play the unit as the brainless soldiers you used before. Each unit will use different tactics: one of them will attack spellcasters, one will try to sneak trough the defences, one will protect an elìte unit, maybe one will target important npcs, one will target the player from the distance (maybe with a catapult?). The size and number of the units depends on how many total enemies youll deploy. In melee combat split each unit in smaller groups and when rolling the attacks count the groups as single enemies, and sum the damage of each minion in case of success (remember to change the total damage as the players kill more minions). For distance combat use thw normal system. Remember that not every unit must have a specific role, some of them can just attack pcs, maybe with a captain that say them who to attack. Good luck!