Go into your key items and click on the jester outfit, it removes it and the Ethan in the mirror disappears.
You need to get her to around 1000 HPs while letting her hurt you a lot (but not enough to kill you, around 150 HPs is best), then have her use Inverse Space so that she exchanges your low health with her. Once you're in Inverse Space, turtle up with healing, Guard, Grab Block, and maybe Insulation too, do not attack until Inverse Space ends. Since she exchanged her health with yours she should have around 450-650, which should be enough to KO her with a Thunder punch before she can re-use Inverse Space.
Basically, you need to trick the trickster.
If she uses Inverse Space before your HPs are low enough, just survive until you're out of it and keep your health low until she re-uses Inverse Space.