This....thing...is....AWESOME!
I believe that is highly unlikely. Just because they are listed as average doesn't mean a creature with above average HP would have the same CR as an average monster. I find that logic very questionable. Are you suggesting changing the HP of an Ancient red dragon from 546 to 812 doesn't make it more challenging? That it shouldn't change the CR? If a group is challenged by CR 24 average red dragon, wouldn't be more challenged by the same dragon, but with 49% more hit points? Now, all I think is needed is a little description of how dialing up or down the HP affect the CR of the monster or the challenge of the encounter (however you want to track it).
O.k., Lets flip this on its head. If you have a party where everyone has rolled under average hit points, say they all have around 49% less, then monsters wold be significantly harder to defeat. Would you increase the Challenge of the monsters due to that?
Do you really consider exploiting what you know about the PCs to be completely wrong? I mean aren’t you already adjusting things based on their level? I mean it shouldn’t be a contest and a DM shouldn’t try to win, but a smart bad guy would try and figure out things and case the players, through Sherlock like intellect sometimes. I can’t do that, but as DM we can simulate that, because we already know things.
What level do you assume this is solo for?
Higher than 13 or so he’s probably not boss fight but a lackey. Still I do say he needs support. Probably exhausted drained PCs at the end of the day and terrain and even lackeys.
Superman can utterly destroy Lex Luthor in a fight.
But does anyone think that's what Lex should do? Get into a fist fight with Superman?
Yeah, the thing is, Superman has a code to not just go break Lex's neck the moment he gives a speech (Snyderverse notwithstanding). PC's are much less inclined to not simply murder a floaty demon head thing.