SERENITY
vs
ALEXISEnter the room, walk a bit forward, cutscene. After cutscene, Serenity automatically makes her way to the boss (unless you press buttons), and finds that the boss is sleeping.
One thing to do: wake her up; it involves a terminal at the top of the room to seal the exit off (and prevent escape).
After that, trying to leave, then return to Alexis and the boss fight begins.
I had a lot of trouble with getting Serenity to automatically run and jump to Alexis, but I managed it at long last!