I don't know that we do. I haven't heard many examples where the dragon swoops down to claw at the party, and the PCs all die because it's just stronger than they are. Most of the anecdotal evidence in this thread is consistent with PCs being uniformly very powerful, and high-level monsters being uniformly chumps.
Okay, fair enough. The examples in this thread have mostly been like that. I’d argue that they’re pretty vague, so it’s hard to know if things could have went differently, and also that it’s mostly a few people citing the weakness of monsters over and over.
I suppose I was thinking back to other threads about this topic, and remembering many anecdotes on the other side. And my own...I have no problem challenging my group of level 15 to 16 PCs. The characters are effective, the players are experienced, and they function really well as a team. The players are all friends who’ve been playing together for a minimum of 20 years.
I think one good encounter I can thibk of took place in the Astral Plane, on the hisk of the dead god Aoskar. The party faced a group of githyanki and their red dragon. I used the base githyanki entry in the MM as the grunts, and bolstered them with a few elite githyanki with fighter levels, a couple of clerics, and a githyanki version of the archmage. Since they were on their home turf, they all had strong positions, including elevation and cover. I played them as an effective group that acted with caution and tact. The dragon was just the icing on the cake. All I did to him was max out his HP.
So I added some class levels to some villains, used terrain and cover to increase challenge, maxed out the dragon’s HP, and didn’t play the villains as though they were just waiting to be killed by the PCs. The githyanki archers readied actions. Their caster used counterspell to foil PC spells. The dragon didn’t sit on its perch taking shot after shot from the Ranger and lamenting its inability to go toe to toe with the party.
Yes, the PCs won. But it was not an easy battle by any means. And although I did adjust some of the bad guys, none of what I did was so drastic as to require a new rule book. I used my own judgment and what the game has already provided.
I could share more anecdotes allng those lines if it would help the conversation to provide examples.
Large groups of enemies still pose a challenge, as do multiple encounters in a day. The traditional dungeon format can be useful for forcing linearity, so that the party has to deal with several challenges in a row, although that can also feel contrived if everything magically resets when they leave to rest. In my case, the party had made a powerful enemy by that point, so they felt obligated to complete each dungeon quickly before anyone could notice and escape to tell the boss.
Also, Empyreans. At one point, I needed stats for the kind of super-powered war golem that a nigh-omnipotent evil wizard would create, and I settled on the Empyrean as the closest thing in the book. One Empyrean makes for an easy fight, and two Empyreans make for a hard fight. Neither of those will challenge a party, on their own, but they can count as one of the six fights in a day. You could probably also use adult dragons for your standard mooks, depending on what makes sense for the world and what's going on.
And in the rare case where I needed a solo monster to challenge the entire party, I resorted to the kind of powerful cosmic beings that I mentioned. In the first case, that was the Archmage/Lich/AncientBlackDragon/DoubleKraken. In the second case, it was a miles-wide dragon of pure energy with thousands of hit points and a breath weapon that could vaporize cities. It's not something you could really pull off in a setting that you planned to use again, but for a one-time event, it provided a definitive conclusion.
Okay. I’ve come up with pretty powerful unique beings for my party to face. Nothing along the lines of what you’re describing, but I homebrewed Duke Amon. Nothjng wrong with that kind of thing. I still had him bolstered by several other creatures, though.
It sounds to me like you are very hesitant to deviate from the established math and the monster entries as they exist, is that right?