High level singular deadly combats suffer two fates: They're overpowered by PCs or the DM contrives ways to prolong them. If the PCs can prepare for the fight they have enough tools that they should be able to modify the rules of the fight to give them the win. This is true of virtually all combats once PCs hit about 13th level - sometimes earlier.
As an example, I had a party of high level PCs with two of the PCs being able to cast Distant Spell Vortex Warp. They would use readied actions to pop in a melee character that could deal 200 damage and then extract the melee PC - before we got to the end of the fighter's turn so no legendary action could be used. Against many foes, that tactic was a nearly impossible to beat winne unless I specifically built to counter it.
You're just not going to be able to contain these high level PCs unless you push contrivance and rules modifications to the point where you invalidate their high level abilities ... and at the point you do that, you're essentially taking the game away from the players and the choices they made in building up their PCs. If a PC invested in Nova Damage and you say, No more than 150 per round matters ... it is essentially telling them they can't have the PC they want to play, right?
To that end, I find that the key to higher level combats and adventures is not narrowing what we allow the PCs to do, but instead broadening their goals so that they have more than just survival and killing to focus upon. Give them things to protect. Give them things to stop. Give them environmental concerns to manage. Make the battle less about the Big Bad and increase the things the PCs need to consider. I try to use this at every level, but it become a bigger piece of the puzzle at higher levels.
You might counter my suggestion by saying that adding a thing to protect, a ritual to stop, or environmental concerns to manage is also limiting them. I agree. They can't use the abilities they gathered in the way they want to use them without allowing this other concern to fail. However, this conflicting priority is different than adding a mechanic to require limits on the damage that can be meaningfully dealt in that the conflicting priority can be story driven and part of the design of the encounter from a storytelling perspective. It adds to the epic-ness of the moment by adding to the tale ... rather than just making the combat prolonged.
But can't the PCs just focus on the enemy first and then turn to the secondary concerns? They can, but then there can be story ramifications for those choices. That is easier to do in longer campaigns where PCs have an investment in elements of the campaign, but it isn't that hard in shorter campaigns / one shots if you work with the players.
This approach has worked for me for decades. It is one of the reasons why my higher level campaigns tend to continue to work much longer than other people's games.
