Well I did it:
And indeed there is no other way around this fight than either stacking an inordinate amount of save bonuses or magic immunity (notice that Desi has now shifted to saying "all Unfair parties should be able to make that save" rather than claiming it'll just get easier or harder "depending on your party's strengths and weaknesses" - which is funny because again, my only claim was that that was indeed the case with every other fight in the game, whereas this one requires something very specific, so QED).
How I did it:
1) Stack saves on my MC, here's the breakdown:
Con: +8 (thanks to Clutch of Corruption)
Haramaki with Sacred bonus: +4
Light Aura from Hand of the Inheritor Shield: +2
Class Bonus: +6
Transformation (thanks a ton to
proxon!!! instead of useless posturing, you actually provided useful information
): +5
Heroism: +4
Resistance: +6
Food: +3
Ring of Summoning: +2
Vision of Madness: +9
Ring of Luck thingy: +1
Total: +50.
Definitely not attainable except by very specific means. Now, what's important to notice here is that
even with Vision of Madness, I'm still close to 50/50 (!), at least without Fortune + Cackle cheese which I'm not using, because it's basically cheating.
Additional requirements:
2) I had to add third impossible domain on Camellia for Madness
2.5) As well, Domain Zealot helps here because it effectively means Visions of Madness lasts longer
3) I got Last Stand on
everybody
4) I thanked The Ghost of Grunkers Past immensely for randomly putting the +1 luck ring in personal stash
All in all, I personally consider this the hardest encounter, because it's literally the only one that need you to something this incredibly specific and warp builds specifically around the fight. Especially Vision of Madness seem mandatory here if you're not a High Fort class.
The only possible alternative I can maybe see you doing is the magic immunity-route, but I think it's actually harder than just stacking saves, because it requires you to find a way to kill Nocticula AND Shamira without buffs - and the Shamira fight follows directly after (combat stops, then starts again immediately).