You must finish the last scene without going into low health (being in the last quarter of life or lower). You must fight darkness infused characters from the story. Dr. Hartman, the Anderson brothers (both at once), and Barry. They are the in the form of the "fast" taken, that go blurry while moving quickly to different areas around you. Crows will also attack you. Use flares if crows are attacking and do your best to dispose of the enemies quickly. Barry is the last of the encounter, and you will have to essentially defeat him 3 times. If you do fall into low health, I advise to reload your checkpoint and try again.