White room simulations are a lot of fun, but they don’t represent how monsters work in actual modules/adventures. Boss monsters usually occupy a dungeon, have a purpose that must be put to a stop, and have NPCs who might create a much more entertaining set of outcome consequences then: party win/party die.
If a white room situation cannot be generated where a higher level monster can not be steamrolled, then the game is probably tuned way too hard for that monster to be encountered in the slop fest of actual play.
A good GM, picking an epic monster as their campaign ended will both Telegraph that monster, give the PCs the opportunity to research it and prepare for it, but also give the creature some unexpected aces up their sleeve/complications that the party will have to problem solve around as they progress towards the final encounter of the final dungeon.
I was viewing the thread on Tree razor and I was wandering haabe any gms thrown a level 20 party at tree razror or the Jabberwocky and how did it go? Did the party win was it a one sides massacre?
A level 20 party can burn through treerazer assuming you have a good grasp of basic tactics (you really should by that point). Ironically, something like the Jabberwocky is more difficult because your standard loadout will probably never include a vorpal weapon, but may include holy given how the odds of an evil enemy are in your favor and a lot of things are vulnerable to it.
As always, fights depend on your party composition, but I've seen treerazer pretty easily beaten by fighter, cleric, bard, wizard. Between hideous laughter, slow and maze, his actions were horribly limited and fighter with holy sword went to town. Everyone else packed holy water as the splash hitting weakness on failure is more reliable than a regular strike. Note the fight was in a whiteroom, with players unaware of the chosen enemy and only having prepped level 20 characters.