-( Story related, cannot be missed )-
Defeat Anamnesis of Chapter 8-1.
You'll see her Aberrant transformation (similar to how Epicurus was Nanana's). In it, she is a pushover to start, simply striking forward with many different swipes. You can easily dash or jump over them. It's when she starts casting that this fight becomes ridiculous.
She will either cast a lancing volley of arrows forward, hoping to hit you if you're far away, or she'll more than likely start casting a Debuff Shower, where she'll raise her wand/staff up and a homing spiral color coded will be chasing after you for a few seconds. The second attack she'll do far more frequently as her health drops, so hopefully Nanana is buffed up or you're fine with waiting for an opening, getting one or two hits in, and then running away. You can always equip some debuff protection Sentiments to make it easier for yourself, or have a fully stocked inventory of debuff cancelling foods on Auto. Either way, once you've completely taken her health down, you'll have finally helped Kokoro get revenge... or will you?