Discussed and averted in An Entry With A Bang!. It takes a lot of Clancy-Earth planes working together to bring down one BattleTech aerospace fighter, and that's only because they were fighting separately. The analysts realise that against a proper House or Comstar ASF unit trained to fight together, C-Earth will be in big trouble.
Played and lampshaded over and over again in Uninvited Guests, not only with actual ninjas, but with the Espada.
Averted in A New Order by the Dark Kingdom. When the senshi start getting more competent they send multiple youma at once, which makes the battles harder.
Done almost literally in one Naruto fic. Nagato, using more efficient chakra receivers and a mind-controlled Shukaku, invades Konoha with 200 Paths of Pain. However, despite having the chakra to power them all, the bodies are lower quality and Nagato can only split his attention so many ways. So instead of six invaders each easily kage level, it becomes more a battalion of elite jounin.
The Dashverse: The Mane Six have a much easier time fighting the amassed forces of Sombra's minions once they're stormed his castle than they did fighting the smaller groups they'd encountered on the way there.