This takes place in the mission Self-Reflection. There is a case file you can pick up on the way to the Prime Candidate Program that gives you the mission.
Fast travel to Ritual Division and go into the Synchronicity Lab. Head to the mirror lab and open the following doors:
Left side: 2nd and 3rd from entrance
Right side: Just the 1st one
Now interact with the mirror, go back into the previous room, and find the various esseJ's.
When the boss battle starts, you can just turn on 1-hit kills in Options to end it quick, or if you want to win legit, just continually launch objects at her and sprint around in circles until you recharge.
Cleanse the mirror to finish the mission.