If you miss this achievement within the story mode you can play challenge mode to take on a Titan in the extreme combat challenges.
This is going to require some patience. Once you first encounter the Titans you need to defeat them using only your fists. If you use the Batarang you will void yourself of earning this achievement.
Once you start fighting the Titans make sure they run into each other and dodge out the way. Once a Titan is worn down enough and holding it's head start attacking. Make sure to evade side swipes, you may need to attempt this more than once. You don't need to kill both Titans just one.