so just for fun, I decided to go at it again with Reimu 94, as I had the save loaded up anyway.
I believe I did 4 tries total, 2 of them being auto quits because Tenshi got hit by Rasetsu fist as the first action each time.
My first try I wasn't sure how fast my characters were, and for some reason didn't buff attack with Ran. It ended somewhere after 4 phases, because I couldn't do damage.
My fourth run was the victory one.
Overall, I lost two characters, being Aya and Kaguya, who both got caught in a Heavenly Demise when I slipped up a switch during Nature form.
My full party was Tenshi, Meiling, Reimu, Ran, Nitori, Flandre, Suwako, Marisa, Kaguya, Aya, Chen, and Alice.
The only difficulty in the fight was making sure I didn't lose anyone to Rasetsu fist in the first phase, and avoiding a double Heavenly Demise before I could get my defense up. I used a lot of swapping to get around big delays, and generally had Reimu buff up my tanking team (Tenshi, Meiling, Reimu, and Ran) while Ran buffed up my attackers. It was worth noting, with around 72% def/mnd, my tank team took 0 from Heavenly Demise (cept for Ran who took 1k or so).
I got 3 of Ran's buffs off before I triggered Rinnosuke's form change. That allowed Nitori to do 100k with Megawatt and Flandre did around 90k with Starbow. Generally I swapped Ran and Reimu in if I missed delay (this happened on two of the six elemental forms) and had them tank some hits, and rebuff while my tanks did nothing but swap (or focus if I was waiting to tank a hit).
Suwako's nuke did a very impressive 150k to the wind form, Marisa's Master spark buffed at 100% did 240k to the spirit form. Alice did 90k a shot to the water form, Kaguya did something like 100k that much to the Mystic form, and I don't really remember what else I did there.
I also spent downtime between forms (when i did enough to trigger a form change) to do a bit of buffing and to try and swap in attackers that might be useful for the next form (generally Flandre and Nitori). Fire form wasn't a big deal either, I used NItori and Flandre
On nature form, I accidentally switched Kaguya in the second slot instead of Aya, and decided to wing it anyway. The result was a Heavenly Demise which killed Aya and Kaguya. Aya actually had enough hp to tank one, but she had been hit by one earlier, so it wasn't very good.
The final form wasn't really noteworthy either. I made sure to save enough sp on nitori for one megawatt, so she got that off, I also got off a starbow break, tanked an attack and swapped to Chen, who killed him with 1 Idaten.
Overall I was a bit lucky and Tenshi never got hit by Rasetsu fist (instead Meiling was hit).
I think it's doable at 90, I don't think 4 levels would've made a very large difference as I never ran into any close calls, my only deaths were due to swapping the wrong person in, and my tank team could survive attacks with a large margin (except for Rasetsu fist on Tenshi) granted I did luck out at the first phase and didn't get double Heavenly Demise before I got buffs up. Also, every person on my non-tank team (except for Aya and possibly Alice) could die in one hit from Heavenly Demise, so I was also lucky there as I missed switching out in time twice.
EDIT: thinking about it a bit more, 4 levels probably did make the difference between sweeping a form in one turn with my equipment setup. I think it's worth noting that I geared everyone except Nitori, Marisa, and Flandre with defensive and affinity gear rather than pure damage (I had 5 blue sabers though, which I didn't even allocate that well as 3 went on Chen who only attacked once). Maybe at 90, with damage gear, maybe the form sweeping speed would be the same.