I am also at the same point, all events save for one secret event are completed. I read on the wiki that there is a secret event in which if you are wearing Glace's armor and are in Celestian form when Glace takes over, then she will learn about Celestian form and be able to use it, and you will also be able to use it when fighting her.
But I don't see how it is possible to have her take over during Celestian form because it only functions while time is paused, but she only takes over if time is passed. I managed to get what might be the closest possible to that: I ended combat while in Celestian form and immediately after shifting back to human form, Glace took over. She didn't say anything about the form. I don't see how she could use it in combat since she always brings Evie along. I was not able to use Celestian form in combat against her even though we were alone in a room together. So it seems that I have failed to get this secret event.
https://townofmagic.com/wiki/index.php/Glace