Said it before and I'll say it again... a "Boss Monster" is a NARRATIVE conceit, not a board game one.  If a DM wants and needs a monster to "last a long time in a fight because the fight needs to be dramatic"... then you as a DM have a STORY you are trying to help get across to, with, and for your players.  And at that point... there is no reason not to use NARRATIVE methods for keeping the "Boss Monster" alive... rather than continually trying to tweak the board game rules to somehow accomplish it.
The board game rules are not designed to create NARRATIVELY-FULFILLING fights and encounters. They are designed to be what all board games are... two sides playing against each other TRYING TO WIN. To get one side down to zero and removed from the fight. And if that is what you want... encounters where you use the D&D tactical combat rules to try and "win"... then it does not matter how quickly the encounter ends. If you as a party can stun-lock the most powerful monster in the encounter and kill it in a single round or two... then you've done exactly what you wanted and what the rules are designed to do. You've beaten your opponent and won the fight. That's the way the board game has been built to play.
But as soon as you as the DM say "I want the most powerful monster to last at least six rounds, and for at least a couple members of the party to get knocked down to 0 HP and have to come back from that, and for there to be ups and downs in the fight where the players don't know if they are going to win"... in other words you want the encounter to be "more dramatic" or "more memorable"... you are wanting a narrative result. Not a board game one. So stop trying to get that by just using the rules of the board game! Because those rules are not designed to create dramatic results! You might occasionally luck out and get it in spite of itself... but the board game rules will not guarantee it happening. Which means you, as the DM, need to basically cheat. You need to break the rules of the board game here and there to allow the dramatic tension of the encounter to occur in the manner you have in your head of what makes an effective "Boss Monster" encounter.
Just do it. If you want your Boss Monster encounter to play out a certain way so it is dramatically interesting to you and your players... then you rig the game so it does. There is nothing wrong with that! Especially if you can do it in such a way that the players can't tell when you are. "Boss Monster" on its turn fiddles with a ring on its finger to crack it open and suddenly a wash of healing magic flows over them and they regain 50% of its hit points and thus can now last another two rounds in the fight? Huh. Did the "Boss Monster" always have that ring on? Did the DM have that ring listed on their monster sheet statblock as an item at their disposal? Was that merely a "quantum healing ring" that only showed up because the party wailed on the "Boss Monster" so quickly the fight was going to end in a round and a half and thus be massively anti-climactic as a so-called "final fight"? Maybe. Maybe not. Maybe the DM just pulled that out of their ass to help create the narrative tension in the fight they were hoping for. But it doesn't actually matter. If you are wanting that "final fight" or "Boss Monster" encounter to be narratively interesting and not just a board game fight of "finding out who wins?"... then it's perfectly fine. A narrative action taken to bring about a narrative result.
				
			The board game rules are not designed to create NARRATIVELY-FULFILLING fights and encounters. They are designed to be what all board games are... two sides playing against each other TRYING TO WIN. To get one side down to zero and removed from the fight. And if that is what you want... encounters where you use the D&D tactical combat rules to try and "win"... then it does not matter how quickly the encounter ends. If you as a party can stun-lock the most powerful monster in the encounter and kill it in a single round or two... then you've done exactly what you wanted and what the rules are designed to do. You've beaten your opponent and won the fight. That's the way the board game has been built to play.
But as soon as you as the DM say "I want the most powerful monster to last at least six rounds, and for at least a couple members of the party to get knocked down to 0 HP and have to come back from that, and for there to be ups and downs in the fight where the players don't know if they are going to win"... in other words you want the encounter to be "more dramatic" or "more memorable"... you are wanting a narrative result. Not a board game one. So stop trying to get that by just using the rules of the board game! Because those rules are not designed to create dramatic results! You might occasionally luck out and get it in spite of itself... but the board game rules will not guarantee it happening. Which means you, as the DM, need to basically cheat. You need to break the rules of the board game here and there to allow the dramatic tension of the encounter to occur in the manner you have in your head of what makes an effective "Boss Monster" encounter.
Just do it. If you want your Boss Monster encounter to play out a certain way so it is dramatically interesting to you and your players... then you rig the game so it does. There is nothing wrong with that! Especially if you can do it in such a way that the players can't tell when you are. "Boss Monster" on its turn fiddles with a ring on its finger to crack it open and suddenly a wash of healing magic flows over them and they regain 50% of its hit points and thus can now last another two rounds in the fight? Huh. Did the "Boss Monster" always have that ring on? Did the DM have that ring listed on their monster sheet statblock as an item at their disposal? Was that merely a "quantum healing ring" that only showed up because the party wailed on the "Boss Monster" so quickly the fight was going to end in a round and a half and thus be massively anti-climactic as a so-called "final fight"? Maybe. Maybe not. Maybe the DM just pulled that out of their ass to help create the narrative tension in the fight they were hoping for. But it doesn't actually matter. If you are wanting that "final fight" or "Boss Monster" encounter to be narratively interesting and not just a board game fight of "finding out who wins?"... then it's perfectly fine. A narrative action taken to bring about a narrative result.
 
				 
 
		 
 
		 
	 
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		