Liches are normally intelligent(often very much so) and very driven by a desire to stay alive. Kinda how something becomes a lich. Playing one as less then very smart is setting up a participation trophy scenario. If the thing has a powerful effective spell, not using it so the players can win is being a poor GM in my opinion. This was described as an end of campaign mega boss fight. The GM should have provided MANY clues about the thing's abilities, possible defenses and such. Smart players would do a pre fight planning session and have ways to counter expected lich spells and abilities. Such as NOT being all grouped up and subject to the AOE burst that stripped away much of the party's magic.
@Remathilis, in
their post about the climactic fight against the arch-lich, was talking about the design and the play of the game. I don't think it's any real answer to that post to just assume the status quo for the fiction and the game play.
What I mean by that is that the way spells and magic work (both in general, and in the particular case of this NPC), the way turn-taking works in combat, the way that fleeing in fear is resolved, etc, are all matters of game design. They can be done differently from how 3E D&D happens to do them, and that can change the play experience.
Different game design can also change the importance of planning. I've done a lot of planning-oriented RPGing (mostly in Rolemaster) It's one way to play a game, But it's not the only way, and these days I wouldn't advocate it as the most interesting way.
And when we look at fantasy fiction of fighting arch-liches - and I've got in mind mostly REH Conan stories - they don't really focus on careful planning and prior deployment of spells and other magic. They tend to focus on in-the-moment toughness, insight and heroics. A RPG can be designed so that it produces fiction more like that, and a play experience closer to that also.
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!
There's no reason why the rules of a RPG - what you call "the board game" - can't be written so that, in play, they produce an experience that has a certain dramatic/narrative rhythm. There are lots of examples out there. Even without looking beyond D&D, there is the example of 4e D&D, where the framework for encounter building together with the combat resolution rules pretty reliably provides a "heroic rally" narrative - the basic underlying mechanical design that supports this is that monsters/NPCs have more-or-less all of their "oomph" built into their hit points and attacks; whereas PCs have a lot of their oomph built into abilities that they need to "unlock" (eg healing surges; party synergies; etc).
And you know what your story actually upholds of my opinion? That the D&D board game can be really stupid sometimes and being beholden to the board game rather than the story means a whole lot of crap happens that never should have. Your experience here highlights that.
That was a bad end to an epic story... resulting from a DM thinking they needed to play the board game "to win" since it was the grand finale. Because what else would a person who puts the board game ahead of the characters and their stories supposed to do? Well... we now see exactly what happens when people focus on what I personally believe is the exact wrong thing. People's memories of their time forever tainted because people were too busy "trying to win" rather than merely just experiencing a truly epic climax.
I am sorry you had to go through that. I wish your DM thought about their game a little differently.
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.
My response to this is pretty similar to
@AnotherGuy's above.
And I think the dichotomy that you are putting forward - either follow the game rules, or have the GM "just do it" and make up and narrate fiction, perhaps retroactively constructing some mechanical rationale - is a false one. It's possible to have a RPG where
following the rules will produce a dramatically meaningful experience. That RPG will need to have rules differently from (say) the classic D&D rules found in the original booklets, B/X, and Gygax's AD&D: but in the 40 to 50 years since those rules were published, there's been a lot of design work done.