I have found a risk-free method of grinding on the 18th floor.
I've manipulated speed so that Ran goes first, and Sanae, Youmu and Flandre all go in that order (speeds of 201, 200 and 199 to outspeed all the enemies).
Banquet of the Twelve General Gods
Miracle Fruit on Flandre
God's Slash of Karma Wind
Laevetain
This kills everything, and I can do it twice per trip to the floor. I go through an average of 2 battles per minutes, meaning about an hour of grinding should put me well above the level I need to be at.
and I'm also getting a ton of nice random drops (Blue Saber, Floppy Rabbit Ear Headband...)
http://www.youtube.com/watch?v=RJ9jFkVaSwIvideo example of what I'm doing.
and Reimu has now reached level 97, up from 81 a few days ago.
Going up to 100 before trying Rinnosuke again. Which of his elemental forms is most dangerous? I've managed to beat all of them except SPI because he never pulls it out.