I recently started following a bunch of D&D subreddits (because I'm clearly an old man in this digital age), and a "problem" with 5th edition that I have seen discussed on multiple occasions is the action economy.
The first place I heard it spoken of was on Matt Colville's Youtube channel, in this hour long video, I think.
Colville didn't say it was a "problem" so much as he simply made an observation: 5th Edition D&D math stacks the odds in the PC's favor when it comes to single-opponent boss fights, especially later on in the levels.
Essentially, what makes the difference in 5E combat is how many opportunities each side has to act. Single, powerful monsters have a comparatively limited number of actions to do each round when fighting a party of PCs. Even with the "legendary action/resistance" powers available to the truly dangerous enemies, their ability to capitalize on any advantages in the flow of combat are cut short by the sheer number of PCs that get turns in between each true action the enemy can do.
So let's say you, the DM controlling a dragon, really smash one of the characters with a critical hit. That character is now close to dead...
In the early levels of the game, one of the PCs would have needed to spend their entire turn healing the characters to make sure they survived. This means the dragon is effectively facing one fewer enemies this round. Do that every round and the PCs have a dire situation on their hands.
But in the later levels of the game, the party can save that PC using only bonus actions (namely Healing Word). So even with the same level of damage output, the monster in this circumstance has a much lower chance of winning a straight fight because it is (effectively) fighting an extra PC every round.
The legendary actions allow the DM, in this case, to have the dragon do a tail swipe or some such attack out of turn to try and further complicate that PC's life and, hopefully, take a PC out of the fight temporarily to relieve some of the pressure.
But there are other ways to slow down the PC's action economy. Here are a bunch, briefly listed:
- A flying creature battles the PCs on a crumbling mountain side. Random PCs must Dex save each round or slide down 10ft. The monster regularly tries buffetting the PCs off balance to inflict falling damage.
- A hyper intelligent enemy has an ever-shifting rubiks-cube-esque boss chamber that it can command to move with a thought. Every round the PCs are slid into walls or each other, out of formation and into spell areas, etc.
- The BBEG runs away, with the intent of being chased, to lure the party into a secondary trap/encounter/difficult terrain, etc.
- The bad guy is rather alchemically inclined, and throws tanglefoot bags and alchemists fire and acid vials as bonus actions.
- The BBEG has a dancing weapon that they reveal in the second round of the fight. Now there are two sources of attacks each round.
- The evil wizard casts polymorph on his familiar in the first round of combat and makes it into a crazy monster.
- The fight occurs in a room full of portals that interconnect. Run into one and you pop out of another. Only the BBEG knows which one leads where.
- Your smaller evil guys can stealth! If they hide behind a tree, they can move away and attack again from a better vantage point.
- Make use of semi-permanent choke points. The bad guy leads the party into a tight alleyway, but faster and athletic characters can take a different side street or climb up a building's siding. Either way, it distracts certain PCs for a round or two.
- The fight occurs in the swamp, which is waist-high water (difficult terrain for medium creatures, swimming only for small ones) that doesn't slow down the BBEG cuz he lives here.
- Start the fight with the PCs in a hole or down a level or two from the bad guy. Let the BBEG shoot magic or arrows or bombs down at them as they climb for the first couple rounds.
- Big enemies grab PCs. They grab them all the time and throw them into other PCs. They use PCs like makeshift clubs, or toss them into spiked pits or out windows, etc.
- Make the point of the fight to stop a ritual or destroy an item, so that if the PCs focus completely on dealing damage to the monster they will fail the main goal.
You get the drift. Change the playing field every round. Keep the PC's on their toes. The monster rules will not give you everything the villain should have at their disposal. If your PCs are using crazy ideas and tactics, your villains should too.
And ultimately, the easiest way to make sure that your PCs don't trounce a big baddie because they get waaaaaay more actions each round is to add more enemies. Evil duos are awesome. Evil leaders (bard, clerics, etc.) with a small battalion of mooks are awesome. A wizard with a golem for protection and a pet chimera makes sense, and makes for crazy combat.
The books have the rules in them, but the real challenges you present to your players can't be found in the books. The challenge is in the way you design the fight and force your players to shift gears multiple times before it's over.