You may expect this to unlock when you first fight the Titans but you will earn it later in the game. On your patch you will be taken to the underground sewers which you need to escape. Joker will force you to fight a horde of enemies as well as a single Titan. Throw a quick Batarang and dodge out the way until you have worn down the Titan.
Climb on his back by pressing to throw a few punches and then climb on, control the Titan with the + and knock down as many enemies as possible before the Titan throws you off. You should get this once you knock down four or five guys.