You will fight Deathstroke during a story mission, early in the game. Every time he attacks, you must press Y to counter him. Attack Deathstroke with Batman's normal strike. Doing so may initiate a QTE where you need to press the counter-button when it is displayed on the screen. If you press the button too early, or too late, your counter attack will fail, and you will get hit. If you get hit during the fight, pause the game, and choose the "Restart" option. The entire fight will start from the beginning, and you can try again. Note: It is possible to miss the Deathstroke fight in the story. If you miss the fight, you must start a new game and play until you reach the Deathstroke Boss fight to get the "Point Counter-Point" achievement.