How not to be stunned forever

My group has completed the Tomb of Horrors.  The second half of the dungeon was even more absurd than the first half, if that is possible.  We crushed our way through it avoiding nearly all of the traps by holding our breath, touching things with magical forces instead of our bodies, and having trap and magic scanning going at all times.  We came extremely close to losing someone to a ridiculous crushing room trap, but thankfully the monk's reflexes were enough to save us.

Finally we came up against the lich at the bottom of the dungeon.  Time for an epic battle!  It was a battle we certainly should lose, by the numbers.  The lich in its lair is a CR 23 encounter, and our group is only level 12.  However, we have a few things going for us that aren't at all normal.  Our rogue had a sword that does extra damage to undead and converts all the damage dealt to radiant damage.  This blade is ridiculous because it bypasses all weapon damage reduction and just ruins undead, like a lich.  We also had multiple characters with the lucky feat because we knew that nasty saving throws were going to happen and we wanted rerolls on those.

The lich led off by instantly killing one character and fearing the rest.  Our rogue chopped in, hitting for half of the lich's hit points in a single attack, so the lich fired a DC 19 Charisma save at the rogue, and the resulting failure meant the rogue was disintegrated.  On round 2 we had half the party dead!  

As I was playing a cleric I decided to just pour on the radiant damage attacks, but this was a foolish endeavour.  The lich, like many legendary opponents, has the ability to ignore three failed saving throws and choose to make them instead.  It also has the ability that if it makes a saving throw, it won't take any damage from the attack that hit it.  Unfortunately we had nobody else forcing saving throws, so the lich was effectively immune to my character.  I could eventually cast enough times at it to get through its three free saves, but it would definitely have killed me by that point.

Thankfully I wasn't actually needed in this fight.  Our melee warlock ran in and bashed the lich to death with a hammer and we won.  One character I managed to bring back from death, the other was gone.  This is an excellent result for a CR 23 at 12th level, but the fight felt extremely frustrating to me.

Here is the thing:  Legendary monsters need some way to avoid being destroyed by save or die attacks.  If they just have normal saving throw numbers, parties will toss endless saves at them and the monster will fail some and be incapacitated for the entire fight.  Slowly beating a monster to death while it is Held / Stunned / Tripped / whatever does not make for a climatic battle scene.  You need to avoid this.  One way would be to simply give legendary monsters outrageous save numbers so they can't fail.  That has the problem that all spells with saves are literally useless, and honestly feels pretty silly.  Players would absolutely have to metagame and refuse to use anything that has a save.  You could also give the monsters incredible hit point totals so they live 20 rounds and the players run out of saves to toss at them, but that makes the fight interminable and wretched.

The solution of 3 free saves though is a pretty crappy one.  If you have a party full of people who toss out saves the opponent will quickly run through their free saves and then get wrecked.  If you have only one person who tosses out save effects though, that person is completely unable to do anything.  The brawlers can do their normal bashing routine, and the people who use attack rolls for spells are fine, but all spells with saves are right out.  That is not a great mechanic at all.

One solution is building a game where groups don't regularly have access to huge numbers of save or be incapacitated effects.  Monks are the worse offenders I have seen, as they can force enemies to save 4 times in a single round.  This usually results in a trivial fight where the big scary boss stands there stunned until it dies.  That just isn't fun.  Sure, the character gets to feel powerful, but 'We wrecked it, super easy' is not the epic story we want to tell.

Another way to do it would be to give legendary monsters advantage on all saves so they get to roll twice.  That is still a huge benefit, and makes saving throw based attacks fairly weak, but it feels so much better to have a chance of doing something instead of no chance at all.  Full immunities suck.  This style would mean that at least you would know that firing off save effects at bosses is unlikely to work, but if it happens to get through it is devastating.  That seems much better to me.

A third approach could be to simply make legendary enemies immune to stun / paralyze and other similarly crushing effects.  Those monsters don't need to be massively resistant to fireball, or being tripped by a fighter, or other similar things.  The real problem is just them being taken out of the fight, and dealing with that directly would be a far better solution than the 3 free saves mechanic.

This whole problem set is just an extension of DnD 5th edition's insistence on going back to nostalgia instead of building something new.  4th edition didn't have this same problem because we didn't have lots of abilities that took people out like we do in 5th.  Monsters got to take actions even if the players really unloaded on them.  I want to fuss at the 5th edition designers about this, because it shows up all over the place.  Unfortunately I have missed the boat on that particular change by a bunch of years now, and I don't see them changing it to suit me.