It all depends how you design your party and what their strengths and weaknesses are.
Let's do some math here to test whether that statement is actually valid in practice:
High fortitude save: +12
Incredibly high Constitution (assuming high base + Aspect of Balor + Tome + Belt etc.): +15
Demongraft: +2
Light Aura from Hand of Inheritor: +2
Lucky Bracers: +1
Heroism: +4
Resistance: +6
Food bonus: +3
Ring of Summons: +2
Vision of Madness: +9
Shake it Off: +4
(Remember, Nocti gets a surprise round, so no Guarded Heath or cheesey run-away-strat for you either)
That's a total of +60. So this hypothetical character stacking basically most save buffs - including some rather niche ones like keeping the Lucky Bracers around for endgame - clears the bar almost exactly (2 points to spare). And it's including what is arguably cheese (Vision having such a low duration, so you have to do some cutscene skips etc.).
It's also assuming a level 20 class with high Fort that has high
base Constitution.
That's a lot of ifs - if any of the above isn't true, things fall apart fast. "Normal" base Con? That's +3 removed there. No high Fort? +4 gone or more. No Shake it Off specifically? +4 more gone.
Now, "clearing the bar", as it were, isn't strictly necessary. If you're OK with reloads, you can make do with 8-9 less than the above. So that leaves you with two options as I see it:
- Have more or less every save buff imaginable but don't use Vision of Madness
- Use specifically Vision of Madness, allowing you to possibly eschew a few of the other bonuses
Remember: Bestow Grace of the Champion doesn't work here, since we're a Demon (i.e. can't be targeted by the spell). Obviously there are other ways to boost saves, including a Demon - we're not hitting a ceiling or anything - but that's not my argument. My argument is that you have to build *specifically* for high saves here, because the fight literally cannot be attempted through other means.
No other fight in the game has requirements this specific - like you say, other fights can be harder of easier depending on your "strengths and weaknesses", but if you built a good party, you'll be able to do them, you'll just have to work harder at it (or, well, cheese it). This fight you straight up cannot do unless you stack an insane amount of save bonuses. As well, it is the only fight I've had so far where both enemy effects penetrate all immunities AND are required to win the fight, period (there are other immunity-penetrating things in the game, but these will hamper you rather than lose the fight outright). Specifically the need for Visions of Madness is part of this reasoning - if you don't have that specific Domain power, the numbers game is wildly against you - especially if you don't have a class with high Fort.
Hence why I think it's the hardest fight of the game (at least so far - I don't know if there are similar "have this specific build or it is near enough impossible"-fights later, only just arrived to Act 5).
One strategy you miiiiight be able to take is to attempt to win the fight with only your MC alive, stacking the saves on him, reloading for the numbers you need. Because the above calculations assume PC (Demongraft etc.). You're going to have a hard time getting near those saves with a full party unless building around clearing a 59DC Fort save on six dude is what you built for.
