~Background Info~
So, aid me, o forumgoers. Point out my ignorance! Who am I to pick, what am I to do?
SPD buffers before the Plus Disk portion of the game are absolutely amazing. I'm honestly surprised you're not using Sakuya or Aya.
In regards to the whole under leveled business:
If Reimu is at or above the recommended level on
the wiki you are definitely not underleveled. Sure a few more levels couldn't hurt but the game's fun when there's a bit of challenge to it.
Most people have a general party they stick with by this point so using Reimu as the basis for comparison is fine as faster leveling characters will have higher levels than recommended but that doesn't mean they're prepared while the inverse is true for slower leveling characters.
I also don't know how the rest of us went about getting through the first form, but I had my set up of Meiling, Remilia, Reimu, Yukari (in that party order) to get through his first form with no losses.
Meiling kept herself healthy, Remilia spammed Spear the Gungnir after a quick Curse of Vlad Tepes, Reimu and Yukari kept the party healthy and defensively buffed up respectively.
I'm just going to summarize the strategy on that page:
Rinnosuke's pattern is that during one of his 6 elemental forms and his final form, every 4th turn he gets (turns 4, 8, 12 etc), he'll be using a strong party wide nuke whose element is based on the form he's in (he uses World Shaking Military Rule during his final form instead of a nuke, which buffs his ATK, DEF, MAG, and MND by 66%).
After 6 turns of being in an elemental form, he has a 50% chance of using Form Shift from then on. Keeping careful count of the number of turns he gets can make bringing out your attackers much easier.
Rinnosuke's FIR Form is the weakest of all his Forms so spend that form renewing buffs and healing your characters instead of trying to kill it. When its the last form left alive, spend as much time as needed to prepare all your characters for his Final Form.
He only has 200k health in this form but obviously, he's the most dangerous in this form so prepare for a damage rush (having Marisa with her MAG buffed up and her SP full for a full power Master Spark helps).
And as Kirin no Sora pointed out, there's still a deal of luck involved as even if you follow the best practices, Rinnosuke might just randomly decide to cut your nukers down with Scarlet Gold Sword / Start of Heavenly Demise. Also, if you haven't already, consider investing into TP for your characters. Both Scarlet Gold Sword and Start of Heavenly Demise take away 1 TP from your characters and you're likely going to be doing a lot of Switches which also consume TP.
Eh, lv90 Reimu can be good for Rinnosuke if you want the fight to be a bit of a challenge and you have a pretty good party. If you go up to Lv100-ish like Kirin said you should be able to wipe it no problem, but, in any case, a few levels can't hurt. I wouldn't recommend using 18f though... the battles are harder and take longer then the previous floors for a pretty small difference in exp/skp gained, IIRC.
"The EXP here is basically
double that of the previous floor, which was itself about
one and a half times that of the floor before it, making this a place where you can make some significant leaps forward in levels... provided you don't mind having to return to base very frequently due to the ability of enemies here to regularly knock out party members fairly easily."
I don't know man, three times the amount of experience you'd get from when you were grinding for Yukari seems like a pretty good deal.