The Doof-Robot is the final stage of the game, and the hardest, if going for this trophy, as the Baseball Launcher is useless against the mech's free-floating hands and does no damage to the chassis. You will only have a small window to fire the baseballs into the core of the robot when it exhausts all its energy reserves. After that window is up, it will activate again and trap you in a ray beam. The characters will switch out. Do the same thing two more times and the trophy will unlock.