It sucks as a DM to have an experience like this. You set up a boss fight, only for it to become anticlimactic as you fail an important save.
I really dislike how many spells can lock a DM out of playing with their own toys. Save or suck spells and abilities are the main problem in this. Many can take a powerful enemy out of the entire fight, turning the encounter into a cake walk.
Even a single round can make the difference, and especially when you are unlucky with your rolls, it can feel like a huge letdown.
Sure, players go through this as well, but they play their character for every encounter in the entire campaign. You usually get to use a cool monster you built up once.
That’s why they added Legendary Resistance. It is a blunt approach that just works. I have seen many cry out for changes to this mechanic, but I am happy with its simplicity and effectiveness.
This is also why there are no more contested checks. Grappling used to circumvent Legendary Resistance, using a single attack action and usually overwhelming odds to lock an enemy out of its movement. This could often make the fight too easy too fast, IMO. I am glad it is no longer as effective.
I have personally found the 2024 encounter building rules with the 2024 monsters to work exactly as advertised. High challenge will always knock a PC out. Being able to rely on the encounter math this way has been a real relief for me.
But it is not perfect. Solo monster encounters still won’t work. When I tested an ancient gold dragon against 3 lvl20 PC’s, it wasn’t close. Command is too strong, and banishment is still a pain. Magic items DO make a difference, and there is no guidance for it.
So if you want a BBEG to be more effective, give it additional LR and double its HP. Read the text of the 2024 encounter building rules. Do none of the descriptions have what your goal is for the encounter? Then break the math, break the rules.
Always expect 2024 characters to have their strongest ability available. The revision has moved away from attrition being something you can rely on, which at least makes it more honest than the 2014 rules.
I have had a lot of success with ignoring the math for certain situations where just killing the enemy is not the goal. Just make a situation that makes sense with the fiction. Sometimes that’s 3 goblins who think they can rob tier 4 characters, sometimes it is a ship full of vampire conquistadors fighting a dinosaur god. Accept that every fight can be a curb stomp or an unexpected PC death, and make sure that you can keep the campaign going in an interesting way as a result of that. Can the enemy flee and come back with a vengeance? Awesome! Now the players have a more personal vendetta too! Did two PC’s die? Make sure you describe their souls entering the afterlife, so you can have a cool scene where a god reveals secrets of the world to the players.
In the end, if the dice are against you, even Tiamat can lose to a halfling burglar. It is up to you to make that story as interesting as her swallowing them whole on turn 1.