OK so here's my take of what's happened/will happen:
- Mayu was probably told by Fueki to take away the Infinity Ring, probably because Infinity is so strong it matches Fueki's power (note how Wizard has never been troubled whilst using Infinity, not even Gremlin was powerful enough, and Gremlin couldn't even touch the White Wizard as well)
- Medusa will probably die next episode by the Holy Ring, she probably goes to Wiseman to seek help, when she sees Wiseman transforming into the White Wizard and telling her it's all his plot, then Medusa goes all crazy (hence why she claims her being White Wizard's good daughter) and dies.
- Koyomi = philosopher's stone, why else would Gremlin say she's his item?
- I think that for some reason, Koyomi doesn't have a phantom after falling into despair, so Fueki needs the Philosopher's stone to heal her, but time is running out as the shell is about to fall apart.