I'll play. If we're talking V5c and only having to survive, I give my Elf Wizard a small chance to solo Nighmare, but it depends upon one thing - Can any of the monsters still find me if I'm invisible?
Room 1 - Take the Dalin path and solve the puzzle
Room 2- You can avoid the fight completely by searching for an alternate passageway
Room 3 - Turn invisible and take no offensive action. Take push damage. What was it? 24? I would make sure my Elf Wizard went into the dungeon a little over 100 hp.
Room 4 - Turn invisible and take no offensive action. Take push damage. I could take out a single wraith in 1 round, but it would significantly reduce my ability to cast spells in room 7.
Room 5 - Turn invisible and take no offensive action. Take push damage. I'm still at about 30 hp.
Room 6 - Solve puzzle. Am I allowed to eat a butterfly in the extra room?
Room 7 - Now I have to fight. I'll probably lose given the necro-knight's damage mitigation, but only because I'll likely run out of time. My saving throws are all at least +29 and I equip Questor's, so the odds are 400 to 1 in my favor of not failing a saving throw. Figurine of Power: Cat will make sure even if that happens I get another chance. His darkrift damage will get reduced to 12 and I can mitigate 14. The question is - can I do enough damage? At this point I've cast Invisibility 3 times (once from card, one spell swap of Ironskin and one from Crown of Expertise. My bonus damage is probably only +30 due to no bard and needing to pump hp. Here's the damage I have left.
Fireball - 30+30 bonus - 15 reduction = 45
Fireball (Ring of Spell Storing) 30+30-15=45
Ray of Shock 18+30-15=33
Acid Ray 12+30-15=27 (x2) = 54 (spell swap Bull's Strength)
Magic Missile 11+30-15=26 (x3) = 78
Shocking Grasp 6+30-15=21 (x2) = 42
Acid Splash 6+30-15=21 (x5) = 105 (spell swap Alertness)
i've assumed a couple of more spell swaps above. That's a total of 402 damage. I think the monster had more hp than that. After that I'm left throwing Thor's hammer. With mitigation, my damage would average 22 (I don't think the necro-knight could be critted). I'm only +14 to hit. I can probably get that up to +16 or +17, but probably no higher without a bard. What was his AC at nightmare? I might need a 20 to hit. If I do, then I'm not sure I would enough rounds in to finish him off. I could use scrolls or wands, but given his damage mitigation, I'm not even sure they would do any damage to him.
What else could I do? I don't think I'd have enough hp to use MEC. I could use Conserve on 1 fireball, so that adds another 45 spell damage. I could throw pixie dust onto the spells. With 16 spells that's another 48 damage. So now I'm up to 495 total spell damage. Sharpen is no good since he can't be critted and Quicken doesn't help at all with total damage. Was he susceptible to anything? I don't think Alter would have helped. Intensify is no good the way it currently works.
Ah, I could have used Conserve on my first 3 Invisibility spells. That would give me 1 more Fireball from the Crown of Expertise, 1 more Ray of Shock from spell swapping Ironskin and 1 more Acid Ray. That's another 124 damage with more pixie dust. Now I'm up to 619 total damage. I'm probably still way short of his nightmare hp. Even if I could tweak my build to get another 5 bonus damage, i'm still only around 700 hp of damage. Maybe I'm getting close enough at that point. I'd probably be better off trying to pump my ranged to hit so that I could get Thor's to hit him at something other than a 20. What was his AC at nightmare?