If they're dedicating 15 of their 24 players holding the generator, why isn't your team absolutely crushing their mechs?

Honestly, 15 of any type will hold the generator. When the opponent dedicates all but 3 of their non-mech players, you let them have it. Use your swarm to eat the mechs and waste away all the ceruleum you're getting, and pick up canisters with a few strays. Their strategy is sound, but it's not invincible. Trying to fight them on equal ground is obviously what loses you that engagement.