Mindlessly ran circles around 20F for three or four hours while watching tv, returned to the 20F boss at around Lv150 Reimu, and took her down without a fight. Time for the plus disc.
As for out little friend on 15F, I don't mind using her, but my team is fairly static. It's been Remi/Reimu/Marisa/8F
since I got 8F, Minoriko before her, and Meiling farther before (in other words, tank/support/mage/healer), and there's rarely a switchout. I don't have a lot of purpose in most of my back 8, the two main backups being 10F
for damage sponge and Minoriko for backup healer. If anyone else comes out, their most likely purpose is to walk out, deal some quick damage, and die, and 15F is okay at that.
And besides, her portrait is cute, so she can have a spot in my party of 12 just for that.