Everyone has trouble with Mystia's second non-spell, don't worry.
Let me try and explain how to do the beginning of IN stage 6b: First, move so that your hitbox is over the "youkai" kanji in the human/youkai bar in the lower left. Once the fairies start firing, inch to the right. When the first fairy dies, move to the left and dodge the remaining bullets once the right one dies.
As for an accomplishment, I just did the above at 600 FPS somehow. :V
EDIT: Ooh, I guess that's one too. From the inevitable resulting run in that stage, I just beat Kaguya no deaths two bombs (well, four since they were both death bombs), bombing only to Buddhist Diamond and Hourai Jewel. I failed her last two spells though. Still, pretty sweet run of Kaguya.