Nightmare Kaiser defeated.
So this took me multiple attempts.
My initial team was basically my usual Mage Meta team with Vivi swapping out for the Healer, because I have his SSB, I figured he makes a decent 5th. So my team would be Terra (top tier Black mage + have her SSB), Vivi (just explained), Lulu (Focus), Edea (Inargural Parade), and Tyro (Sentinel Grimoire for the first 2 phases, also can use stuff like Eiko's Fairy Flute++ for synergy)
My RWs used were Mighty Guard VIII because it's basically perfect for this fight, and Arc's Renewing Rain because with Edea's inargural Parade, I don't need the Hastega that much, and I still get Shellga + Heavy Regen, and the Magic Blink is a nice bonus in this fight. Anything else didn't seem worth it.
My first attempt involved not completely understanding the fight so that didn't work. Learning what elements he has, I swap things around, try to cover everything twice, and I bank on Heavy Regen healing me.
Then Poison starts fucking with that, lots of bad RNG, etc. I also learn you DON'T want to activate the crystal every shift...
...so after multiple failed attempts and tweaks, I forgo Magic Twinking and equip for elemental resistance prioritizing Fire over everything else because Meltdown. In doing so, I can ignore the Crystal in the Fire phase entirely, and I plan on hitting the crystal in the Water and Lightning phases (because I need fight those anyway), and Poison because that's a pain to defend against.
My offense is shakier, and things don't go the way I want. Then I realize the problem with my set up:
I'm not using healing beyond Vivi using Master Mend. Realizing that, I give Tyro Curaja...and then recognize Vivi is basically accomplishing nothing in this fight since he's using his Soul Break on the Medica anyway. So I swap him out for Papalymo. Why Papalymo? First Sage who came to mind and my team was ordered that way. Also I learn summons CAN be used in this fight, so I take advantage of that with Terra somewhat, so my final set up looks like:
Terra: R5 Firaja, R3 Ixion, Devotion
Ixion can be used twice in phase 1, and she can hit both the crystal AND weakness condition in Phase 2 on the last casting and deal a crap ton of damage. She has Devotion because she's my most durable character, she has the SSB that hits hard with both buffs (21k), and Ixion means she can occasionally ignore the damage cap
Lulu: R4 Blizzaja, R4 Thundaja, Vow of Vengeance
Just a cannon who can occasionally use Focus for the stackable Faithga and extra layer of mitigation in the first 2 phases.
Edea: R4 Blizzaja, R4 Bioga, Mako's Might
I made Bioga specifically for this fight, probably wasn't necessary and could have used my already existing Darkra but Bioga did let me activate the Poison crystal. That extra Blizzaja was made for Ifrit, so no, not a typo; mostly just another cannon with an extra Soul Break. Not as good as Lulu because of her RM, but it gets me the immediate hastega.
Papalymo: R5 Curaja, R5 Waterja, Blood of the Espers
Shifts between damage, healing, and hitting that condition; on hindsight, he maybe should have covered a less integral element but overall didn't matter.
Tyro: R5 Curaga, R5 Waterga, Dr. Mog's Teachings
Sentinel Grimoire to power through the first two phases, Waterga just insurance to hit the condition in case something happens to Papalymo, as well as being a 2nd caster for the Crystal so I don't have to rely on Terra wasting a gauge on Trance Flood.
RW was Mighty Guard VIII, don't think it would have made a difference in this fight compared to Renewing Rain, but whatever.
Things went a lot more smoothly with elemental gear, and healing but the randomized conditions are still bullshit and you really are dicking around a lot waiting for the right two elements to drop. I also learned you DON'T want to activate the Ice Crystal because you might kill him Phase 2 when you're weak to Ice. Everyone resisted Fire so activating the Fire crystal was pointless, and the Earth crystal only matters vs. his nightmare more, so I can just have everyone defend during that phase; this is why I decided Lightning/Water/Poison.
Lulu did die in the winning run because he used Nightmare Tsunami and she had the Gigas Armlet...whoops. I say to myself that if I lose THIS time, I'm adjusting it that Terra gets the Gigas Armlet, and equips the Viking's Jacket, forgetting that is a thing I have.
Honestly, I think the fight isn't that bad at it's core, it's just they gave it dumb conditions. They should NEVER have weakness conditions on a Barrier Change boss unless the boss has a legitimate cycle that you can predict. This isn't the first time they've done it, no, but it's always annoying seeing it because you can fail a condition because you are "too good." At least status weaknesses, there are (semi) reliable ways to apply it most of the time, and you aren't dicking around waiting for the right moment. Barrier change conditions, especially on a Nightmare, is bullshit.
Oh well, that's done with, time to make Meltdown.