How were the Giant Hornets actually defeated?
I don't think it was ever stated how exactly they were wiped out, but we do know that it wasn't a WMD (no damage to the surrounding land), it wasn't the fiend that killed them (death feedback), and that the fiend stopped their arrows and disarmed them. It's also possible the fiend prevented the Giant Hornet soldiers from attacking any of the Robber Fly and that's why none of them were killed. Being unable to fight back, all the Robber Fly soldiers had to do was slaughter them all.

That could also be the reason Kiroumaru was the sole survivor of the battle, in order to bring suspicion onto him from the humans. The Robber Fly could have purposely let him live.