S Intermezzo

This episode is a long flashback that tells the tale of Mimiru's mentor, an amazingly powerful character controled by 3 high school girls with the intention of working together to create a extremely powerful character before their graduation, and they pretty much suceed. The novice forms of Mimiru and Bear barely survive in an overpowering dungeon until she comes along, taking out the opposition in a single blow.
Mimiru accompanies this Gardenia look-a-like to the bottom floor of the dungeon where they are faced by a supposedly invincible enemy, so to protect Mimiru, she uses a skill that crystallises her character but guarantees the enemies death.
Mimiru goes to the bottom level of this dungeon with Bear once again to find her mentors form, still preserved perfectly, standing in the exact same position as when she used her fatal skill.

Next episode

S01E28 - In the Case of Mai Minase