She isnt a secret boss. You need to "defeat" her to progress in the story.
All I can say without spoiling is keep an eye on your"Status icon" during battle and change the way you act accordingly. If you wish to know directly how to defeat her. Please ask in the Spoiler topic: https://dmquetzalli.itch.io/delve-into-pawssion/devlog/382260/guides-and-secrets...