F2P. Was stuck on this for almost a month, I think. I looked up as much info as I could and everyone was either vague or had conflicting info. I wasn't sure if I was attacking the right part of the Glass Slippers. Only this guide explained it clearly: • Chapter 32 boss Mirror Container Full...
Anyway turns out I didn't need to destroy all four slippers because Cinderella does so much damage. Skills are 10, 7, 10. Other characters are either max skill or max recommended.
BTW I decided to quit Blue Archive today. It was taking up too much time and I wasn't enjoying it. The only reason I kept playing was because it felt like a waste to stop, since my F2P account had all the meta characters maxed out. But somehow I was always struggling in most of the modes, even if I copied other people's strats. PvP was especially painful. Even if I was way stronger than opponents and copied other people's teams, I still lost a lot. Anyway, I'll probably quit this game next, because it's almost the same situation as Blue Archive was.