minimum 72d8 dc17con save for half from cone of cold alone
Well first you have to explain how you're getting 10 spells out of 6 players in one round.
But even disregarding that, and assuming you targeted the Orcs, your party on average dealt around 324 damage, doubled as its AOE against a Horde, and then someone probably did nothing with Dragons Breath, as even at level 20 it'd be pretty dumb to walk a caster into an Orc Horde to make that touch range spell
You still have around 4100 Orcs to kill, nobody bothered to do anything to the Arch Mages, and here comes the Dragon.
You should also be aware that, especially at level 20, all bets are off as far as death is concerned. If you die, you die.
My players didn't. They took 7 rounds the first time I ran it, with the Hordes breaking on round 6 and the Dragon BAMFing on the 7th. Archmages were killed early as (IIRC) I rolled for naughty word on some of their saves so they death spiraled, which was fitting given how I roleplayed the bunch.
If the encounter requires the GM to run it with one arm behind their back & the other clad in a gentle velvet glove to avoid round one instagib
The encounter requires neither. As stated, the encounter works. My group has not only run it without issue, but done so more than once as I reskinned it for another campaign.
Your assumptions to the contrary are only indicative of you not listening to what Im telling you.