Hiya!
The players get four turns to one turn of the Big Bad Evil Guy. It's fine if, in a combat that lasts three or four rounds, each PC misses once or twice, because overall the party still does something interesting each turn.
I don't think the boss should miss with their attacks, or at the very least they should have an effect regardless of whether their attack hits. This was a common design conceit in 4e, but not in 5e.
Now, in traditional video game RPGs, the PCs and the boss (almost) always hit, unless someone is hit with a condition like blinded. On the other hand, in many action video games the boss will try to do something dangerous, but you can dodge or parry it. However, there's always a sense of the boss being dangerous, and the PCs having to pick the right tactics to survive, rather than just relying on luck of the dice.
What do you think? Should D&D boss monsters have more abilities that don't require a die roll to be threatening?
To be honest, uh....just stop trying to keep the PC's alive all the time so they can get to the BBEG.
I'm not saying you do this, but I have heard this same 'complaint' that the BBEG goes down far too fast. The most common solution given is to "beef up the bad guy and use lackey's". The problem with this is that it doesn't work most of the time. The REAL problem is that the Players get so used to "winning all the time" that if the BBEG fight does start to go against them and they loose/die...they scream bloody murder and accuse the DM of "cheating" or "deliberately killing their PC's" or otherwise creating a "no-win situation".
THAT is the core problem. Not that main villains loose in a few rounds. Make it so that death is VERY real every step of the way. If PC's can, and do, die from a fight with an engaged cave bear...so be it. If they die from a poison needle trap (sorry Blackleaf!)... so be it. Let death be an ACTUAL threat
at all times (to varying degrees), and when the PC's get to the BBEG, they will be sweating bullets every round. I guarantee that!
Then, after a half dozen rounds, and the BBEG finally goes down...you will have a group of very, very happy players at your table. Why? Because they KNEW that, at any second, any round,
something could happen to kill a PC. I mean, Gurdaak the Barbarian was killed several weeks ago with that Cave Bear, remember? And poor Blackleaf got stuck by that poison needle trap and died instantly! Do you think the DM is going to suddenly 'go easy on everyone' because it's the BBEG? Pffft! Not on your life!
So...stop "building encounters to challenge the PC's, but not kill them", and start "building to the world's internal logic and consistency"...and let the dice fall where they may...and suddenly, that 6 round battle with the BBEG is nail-biting, edge-of-your-seat, sweat-drenching horror!
Or keep doing it the 'normal way', and have the Players expect to win, and when you "over build the encounter to kill at least one PC"...you
will have a group of very, very upset players at your table. Why? Because they KNEW you specifically put that power/spell/item on the bad guy in order to 'nullify' their PC's powers. They were expecting to win, becuase you have taught them that they will always 'win', or at least never 'really loose badly'. So the second they DO loose badly, at the BBEG, who happens to have every counter to their abilities...well, honestly, the DM has, effectively, "cheated" at the game. He isn't being fair anymore; he's specifically TRYING to mess with the PC's and Players.
Don't do that.
Sorry that was a bit "off track", but I honestly believe that the problem isn't that BBEG's go down so quickly; it's that the DM wants it to be a big epic fight, but to the players, it's "just the end fight". Nothing to worry about. Ergo, when they win in 6 rounds, its "ho-hum...that didn't take long", because, in their minds, there was never any real doubt. It's not exciting. It's not, well, anything to write home about. Have PC death be a constant, in-your-face, you could die at any second, type of thing throughout every game session....and you now have a COMPLETELY different mindset in your players.
^_^
Paul L. Ming