I'm wrapping up a high level (13-20+) hexcrawl adventure right now. It's ready to go on the DM's Guild as soon as I finish about another 120 pages of editing (bestiary, hex entries, items, page spacing, etc. Bleh).
It has gone pretty well.
-The 5-8 encounter adventuring day DEFINITELY breaks down. I'm seeing one big fight every couple of in-game weeks. Those in-game fights are big and involve multiple enemies with 7th-9th level spells, and time pressure where the party has 6-8 rounds before reinforcements flood the area. Power Word Kill, Finger of Death, Prismatic Wall, Earthquake, etc. are all on the table. Usually I get 1-3 PCs down to 0hp during these fights, but they don't die.
-Strategic initiative matters. Scrying, attacking the PCs when they aren't ready, and evading enemy patrols to avoid 500 enemies jumping the party are all big things.
-Having a pre-populated area with multiple viable goals at any given time means the players can choose "whatever" and all I have to do is look up the right page(s).
-Not every random encounter has to challenge the party. The 19th level monk has made notes of a giant honey tree (lots of bees) for a return trip. As a combat encounter, it'd be challenging for maybe...2nd level players?
-In fact, most things aren't going to seriously exercise the players and put their characters in danger. Instead of "is what's around the corner going to kill me?" it has become "How do we provide evidence to get the Giants on-side so that when we go up against a god's avatar, we're bringing two polities, a kraken, and a nascent god-sword to the fight?" along with "Do we want to try to trap these CR 5 carnivores and try to let 100 of them loose in the enemy city? If so, how?"
Here's the text I have written up on the topic in my "How to run the campaign" chapter:
High level D&D characters, particularly spellcasters, have the ability to survive almost anything, deal massive amounts of damage, and reshape the battlefield in a round or two. At 5th level, a Fireball at the wrong time can cause a TPK. At 15th level, a DEX-heavy party can shrug off 3 Fireballs and a Prismatic Spray with no ill effects.
In combat, this means that the DM is free to throw lots of firepower at the PCs, and trust that they will be able to dismantle it in a few rounds. Reviewing the included Campaign Log will show a typical party handle an invisible ancient dragon with ease, teleport to an enemy airship and wipe out its crew, drop into an enemy temple and kill the high priest, desecrate the altar, then leave, or even split up to conduct hit-and-run raids with a Hasted Monk who can literally outrun the enemy. This may seem like a challenge to DM, but the DM’s job is not to conduct the party’s strategy or tactics – simply to make a good effort at defeating them with the resources on hand. Sometimes, the enemy will scare the PCs or chase them off. Sometimes, the players will get good rolls and will cut through 60 CR worth of opponents like a +3 Flaming Dagger versus warm butter.
Out of combat, high level players have access to extreme strategic mobility, including Scrying, Teleport, and Transport Via Plants. This may seem hard to plan for, but that’s the advantage of a large, pre-populated map. The Aarocokra also have the benefit of a Scrying chamber at every temple, and can be assumed to have good, but not perfect, ability to track the party unless or until Scrying is blocked. The players again do most of the work; the DM simply decides when the players should be attacked, what reasonable steps the enemy is taking in the background, and what additional reinforcements have been dispatched to temples.
The hardest part, in the author’s experience, is choosing quickly what actions enemy should take in large battles (10+ foes vs. the party). Spell selections presented in the Bestiary are typically abbreviated and categorized to help; beyond this – pick a few default actions and use those. Low-level divine casters can’t go wrong with Sacred Flame or Guiding Bolt, and high level casters are likely to use their high-level spells first.
The players do not see what happens behind the screen. Sometimes the DM will forget a creature’s special abilities or make sub-optimal choices. Most of the time, the players will never even know, and that’s okay.