Instructions
Knock out Meryl. Psycho Mantis will begin the fight by psychically controlling your colleague, Meryl. You'll need to punch her (by pressing the "circle" button) repeatedly until she is unconscious, which will cause Psycho Mantis to focus on you.
Pause the game. Unplug the PlayStation controller from the console's player 1 socket and insert it into the player 2 socket. This will prevent Psycho Mantis from predicting your moves, as he can predict any move entered by the player 1 controller.
Avoid the objects that Psycho Mantis will throw at you with his psychokinesis and keep shooting at him with any gun at your disposal until he is defeated.