I just did a two-shot where we went five level 20 characters vs 5 Ancient Dragons. Beyond all expectations the players actually won without a Permadeath. That said, the GM didn't use the Dragons' spell abilities much, that might have turned the tide.
Worth noting that it was the fact we had two Druids with Shapechange that turned the tide of battle.
First Druid Shapeshifted to Ogremoch soaking damage and myself as the second Druid Shapeshifted to Pit Fiend multi-striking with poison helped to finish the fight, our Cleric was putting out some serious dps with Smites though he went down and had to be revived twice, a sorcerer with Invulnerability potion successfully splitting off two of the Dragons with Agro and doing some strong AOE until the Pit Fiend came in to finish them off. I was the Pit Fiend Druid, Dream Circle, focusing healing after summoning a Draconic Spirit to soak damage early on until the Shapeshift towards the end. Wizard in the backline focusing Lightning damage with hasten, but the Blue Dragon being immune he switched it up towards the end.
Damn, that sounds like one long epic fight! Thanks for the rundown. That was an interesting read. I love getting a look into how different people problem-solve stuff like that.
As a bit more background the Ancient Dragons were Blue, Black, Red, Green, and White, so we had to work around a mix of damages. Nice bonus with the Pit Fiend, Dragons aren't immune to the Frightened Condition and creatures within 20 feet of the Fiend roll for Frightened at start of turn, and all attacks are magical so it punches through resistances.