(For what it's worth, I think a party of 5th level adventurers could mop the floor with a single CR9 monster)

From what I can tell, the crux of what you're trying to accomplish here is to have a monster with a very high effective defensive CR, while keeping the offensive CR reasonable. Such a monster will be able to last a long time against the party, but not be able to wipe them easily. But as you can tell, simply jacking up the monster's HP makes for a slog.

Split the fight into multiple encounters/stages

This tactic is a classic trick in video game RPGs: after dealing a certain amount of damage, you change the boss's form or the circumstances of the fight. This allows you to mix up the fight, and effectively turns the fight into multiple consecutive encounters hidden within a single battle.

For example, a party I play in recently fought a homebrewed boss dragon. It started with some chromatic abilities, but after we dealt enough damage to it, it changed into a dragon with all of the chromatic abilities. After that form was defeated, it turned into a shadow dragon. Afterward, the DM said that he just chose a CR-appropriate dragon, and then added not-so-powerful abilities to it for each stage. The resulting fight was both long and difficult, but not too difficult.

In my own game, I have used a boss that starts in a human form, then turns into a dragon form, then into a more powerful dragon form. Alternatively, you could have the boss retreat to a different location--perhaps one where he has traps set up or some terrain-based advantage.

One nice trick is that you can use the form change to clear otherwise debilitating effects. Almost every single-monster fight ends up being lopsided because of the imbalance in the action economy, but having your monster change forms gives you an excuse to get rid of that debilitating status effect (stun? sleep?) or allow it to survive a massive nova (a paladin dropping a ton of smites in one round of attacks, for example). At the same time, it doesn't render the players' actions completely useless, because they still made that stage of the fight easier.

Give the monster an interesting defense

If you don't want to have changing forms, you could try giving your monster interesting ways of keeping itself alive.

For example, consider the Archmage (MM 342). While he technically has access to 9th level spells, none of them directly deal damage. In fact, his highest level damage-dealing spell is the 5th level cone of cold, and all of his spells of 6th level or higher are defensive. Thus, he has a great deal of potential survivability, without being able to wipe the party in one shot (which he could easily do if he had meteor swarm instead of time stop). In my game, I had a fight where an archmage could form a huge slime armor around himself, which gave him extra tentacle attacks and a shield the players had to bypass first. Thus, while boss's damage output wasn't too high, it became an interesting puzzle for the players to pierce his defenses.

In the end, the PCs are always going to be super death-dealing machines. In order to have an extended, meaningful fight, you have to find some way to make their damage output materially useful during the fight e have some way for the boss to survive that damage output. The fastest way to a boring fight is for the players to feel like their attacks aren't making a difference, until the boss suddenly drops dead.

What things do I need to take into account when constructing an engagement that is intended to endure for a long duration (7-10 rounds), against a single creature, with the intention of not killing the party?

Let me first say that personally, I don't see 7-10 turns as very long. But my groups are smaller (4 PCs) and lower level (Started at 1, currently 8) and a bad round of dicerolls can happen any time.

The key to fun is decisões.

A player making a meaningful decision is a happy player. A player that has no choice to make is a bored player. The first decision the players made was to lutar at all. Then what? "I attack" is not a decision. Que decision was made before turn 1.

To enable players to make decisions todo turno ou pelo menos every other turn to account for the fact that a bad diceroll might mean the decision only takes effect after a while means the combat has to be somewhat dynamic. If there is one obvious, optimal strategy, then there is no decision to be made. You roll the dice. You see the strategy unfold. Turn after turn. More like a video than a game.

So how to have decisions to make every turn?

Don't allow for an obvious ideal estratégia.

Take for example a dragon: the breath weapon means it's dangerous to stand close together. However, if they all split up, the dragon with it's better mobility might single one out and the PCs might, because they are melee or because there is cover in between, not be able to attack the dragon when spread out. So every round they need to decide whether spreading out is good, or maybe not.

The dragon is just an example. It would be perfectly boring if the dragon just sat in the middle of a plain and let everyone hit it from all sides. You could have all players formulate their optimal strategy in advance. Write it on a napkin, assign a diceroller and come back when he's done to get to know the result.

There certainly is no silver bullet, but there are many monsters (or combinations) where a combat is dinâmico instead of static. Add terrain as necessary to broaden the choices players can make. But make sure it's escolhas. "Do I want to stand on high ground for +1 to hit, or not?" is não a choice. "Do I want to stand on high ground for +1 to hit, when it also increases the chance of being struck by the lightning the druid summoned?" is a choice. A non-trivial decisão to make.

Otherwise... practice makes perfect. Try a few and adjust when necessary.

Better question: how to make an engaging encounter?

You make a high level encounter exciting by making it an exciting encounter to begin with, regardless of the level of the fight. Go back to the basics of encounter design, and once you have those set, customize them with some high level twists.

You can build a good high level monster fight by making it a fundamentally good fight first. The high level monster bit is just icing on the cake.

A really good resource in this regard is the AngryGM's Como criar Encontros impressionantes em F $ e% ing!, which outlines what pieces any encounter should have, combat or no.

I'll get to his suggestions later. Below are my own.

Manage the pace properly

Your encounter has to be a roller coaster ride. Create moments of rising and falling tension while adjusing how fast you move between them. You must plan the moments when the party should lose all hope, and the moments they regain it. There should be multiple dips and rises in one encounter. You must also create the environment for them to occur naturally.

In general, your pacing curve for an encounter should look like this:

1. Open big with an exciting sequence. This is the party delivering their surprise round (action); or the moment the party is ambushed (suspense); the death of the princess they were escorting (sorrow); or the revelation of a dark secret that changes the PCs' perspective (tension). This is the spectacle you create to draw the players' attention.

2. Oscillate between high and low action in a steadily increasing manner. Follow your opening scene up with a more somber moment to set the players' baseline expectation of the action, and slowly but surely raise that baseline.

   This means filling the room with less powerful minions, activating some traps that slowly get deadlier, and providing temporary shelters/reprieves that dampen the excitement of the previous sequences.

   You create moments of release to give the players time to relax. Otherwise, they will acclimate to the intensity of the experience, and it will be harder to get them to engage. Crush their spirits, but give them hope, too.

3. Remember to follow this curve on a round-by-round and turn-by-turn basis. A really good example of this is episode 102 from Critical Role.

   The BBEG delivers a Finger of Death to the cleric with the possibility of killing her (low point), but the bard counterspells it (high point), and then rolls a natural 1 (low point).

   You can see the players' reactions as their spirits rise and fall in the space of one turn as the tension oscillates, and one of them even says "What is this up and down cardiac arrest of a train?"

   That is what you want to hear. That is what you want to recreate. The players are so engaged, they feel like they're having a cardiac arrest.

A bit more in-depth explanation is provided by this guy on Extra Credits in the context of video games. To get the gist, begin at 0:30, watch until 1:10. To get a better idea, watch the full 6 minute video.

AngryGM's Ideas

I won't give you the full write-up of what Angry says on his article, as you should probably read it yourself, but here's the summary:

1. Start with an idea, or high concept. This is something like "an encounter with a three-headed dragon guarding a princess in a tower" or "an encounter against a hydra under the water." This idea is your foundation for the whole thing.

2. Propose a dramatic question. These are things like: "Will the heroes save the princess from the dragon?" or "Will they escape from the collapsing tower?" This is the question about whether the heroes/PCs can accomplish their metas. It has to be a statement about what the heroes want to accomplish. When this question is answered, your encounter has ended.

3. Incite a call to action. In case your players don't want to answer the dramatic question, you need to pull them in with your hook. For example, you could take a whole day planning out an underground dungeon with the question in mind: "Can the heroes survive entering this dungeon?"

   But when the PCs see it, they can reject your question and propose their own: "How can I destroy this dungeon without stepping foot inside it?" And there goes your entire afternoon. But if they had a call to step inside -- like a person to rescue, from a quest -- then they would tem to accept your question.

4. Provide conflict. Now that your PCs want to achieve a goal, you have to stop them from achieving it. This is where your high level monster comes in. The monster wants to stop the heroes from achieving their goals. So if the PCs want to save the princess, the monster wants to keep her to itself. Note that putting a monster between the PCs and their goals is not a source of conflict in itself. If the monster has no reason to oppose the PCs, then there is no reason for the fight to occur.

O artigo vinculado has even more information than this. It's really long. It discusses ending an encounter, creating and intensifying "decision points", mechanical ways to implement this, and an example applying these concepts.

However, strictly speaking, you only need the four points above to create a good and complete encounter.

Making it a high level battle: just throw in the high level BBEG

As long as you follow the rising and falling pace of a good encounter, plus incorporate the basic elements of encounter building, you should come out with a gripping fight. This is very bare bones though, and the specific encounter you make will depend on the monsters and ideas you want to implement.

One high level fight I enjoyed running was against a group of five level 13 PCs against a level 20 Paladin/Sorcerer. The fight went something like this:

1. The players enter the room, ambushing the Sorcadin in his sleep.

2. They start hammering down on him. He retaliates. Damage is traded back and forth.

3. The PCs start going down. The healers have to bring them back up. They continue to trade damage.

4. One of the players start sweating, notices how imbalanced the scales are, and says this was a mistake. They should run.

5. They continue fighting.

6. The Sorcadin falls unconscious.

7. They don't kill him. He wakes up while they're restraining him. He casts piscar with Subtle Spell and escapes.

This was the final boss fight in that campaign. They defeated him, accomplishing a specific goal the entire party shared. All the players, without being asked, said it was a great fight.

Conclusion: build a great encounter first

All that changes is the scale of the fight at higher levels, but no amount of mechanical tricks will fully replace a well-designed encounter that makes full use of our storytelling tools. Every fight, every encounter, and every campaign should follow alguns pacing curve that was consciously planned by the DM, to ensure maximum chances of engagement.

If you do this naively, the problem you'll run into is that your monster will focus fire on one PC and keep attacking until that PC dies. If that happens, it's too deadly for the one PC who took all the damage, and it's boring for everyone else who never gets hit.

So the most important thing to do is to make sure your monster distributes damage among the party. Perhaps you can give your monster some AoE attacks or spells, or perhaps you can make sure there's something in the environment that makes it attack different things.

Many people will argue for 'scripted' battles, where the monster takes on multiple forms. That's a good way to do things. Another good option is to add some minions to the battle -- maybe one minion joins the battle per turn, or something of that sort.

Lair actions are another good way to make sure damage is spread out among the party.

Firstly, there has to be a real possibility of danger. As the DM you shouldn't favor the players -- that should be built-in to the game mechanics and properly describing the scene ("You see the monster looming over you (i.e. it's tall) holding a 4ft broadsword (armed and within reach). His eyes show awareness of your every move.").

Secondly, a death match isn't the only dynamic between conflicting parties. One could bring one to certain death only to back off and strike a deal. OR, they may flee and take the PCs to a secret lair full of goods that they otherwise wouldn't encounter.

Thirdly, you could really think about what different tipos of NPCs do when pushed into battle -- they aren't all going to be "fight to the death" battles. In fact, perhaps most are not. So consider thier intelligence, their (lack of) strength, their constitution (a low constitution NPC will probably try to end the fight early by surrendering "Okay, I give up, what do you want?"), etc.

Fourthly, consider the battle from the NPC's self-interest: do those players of yours have some nice equipment? Might eles be trespassing and ASKING for trouble? And one of them looks a little tasty doesn't it?

Espero que ajude.