I had a huge strategy guide for the 18F boss that I spent 40 minutes typing up...and then I misclicked and lost it. >_<
So let me just try to say this quickly: protecting against status effects is actually quite important in this fight. 3 of his 4 unique spells in the Wind form cause paralysis, half of his unique spells in both Wind and Mystic form will debuff the whole party, both Cold and Spirit form get multi-target silence effects, etc. I daresay that guarding against Silence and Paralysis is more important than equipping gear to boost other stats, since those effects are effectively a OHKO. Paradoxically, instant death isn't as important to protect against, since he's only got one spell in his Spirit form to cause it, with only a 5% chance of using it each turn, and it has a relatively low proc rate for instant death on top of that. Poison is also probably not worth guarding against, since only one form uses it, and you can probably kill it before that form kills you. As for debuffs, well...you can try guarding against it, or you can just use characters like Iku and Reimu to buff your party again.
To reiterate, guarding against Paralysis and Silence is important for this fight. Characters with Nature attacks (Youmu, Kaguya, Suwako, etc.) should focus on protecting against PAR, while those with Fire or Mystic attacks should focus on SIL (although the Spirit form can be ripped apart by a fully buffed Master Spark, if you have it).
For the final form, note that it has relatively low HP, but increased resistance to all elements. That makes non-elemental attacks the most effective here. Slash of Eternity, Megawatt Linear Gun, and Starbow Break can all expect to take out at least a quarter of the form's HP even if unbuffed. Silence Selene, Flight of Idaten, Soaring En no Ozuno, Spear the Gungnir, etc., can also put up good results.
Even Sakuya's Killing Doll does 50k damage on this form when backed by Thundercloud Stickleback. I usually don't have much difficulty with the boss's final form, because I tend to keep my party alive and fully buffed due to being extra-cautious with my setup and strategy with the other forms. Sure, I might lose characters on his final form, but I just keep throwing the strongest attacks I've got and win quickly anyway. The form only has 200k HP, and my buffed non-elemental attacks do 60k-90k damage each even at Reimu level 90, so if the boss tries to go for World-Shaking Military Rule followed by Start of Heavenly Demise, he usually lasts only 1 turn in this form anyway. Unbuffed SoHD isn't strong enough to cause a party wipe since it's composite damage.
So bottom line: for an easier time with the final form, keep your non-elemental attackers alive and buffed through the other forms.
I personally don't think you need to be that high leveled to take down the boss. I usually fight him with most of my party in the 80's.
