When you're in the Botanical Gardens following fingerprints, and you reach the part where one henchman is in a control tower, and 3 others are in other places watching out for you. Make your way to the man in the tower and take him out. Take out the other henchmen, and the proceed to find the secret door. This should be easy in detective mode, because the door is where the fingerprint trail ends and should have an action button (A) to open it.
Once you're in here, you'll have a cutscene with Joker and two titans. When this ends you have to defeat both titans. Once one of their bars has 1/3 taken off of it, you can then hop on the back of one and start punching the other. Use your batarang to slow them down when they're rushing at you, and dodge. Just use the same tactics as you did on the earlier titans, and be more wary since there are 2 of them in the room.
(This would be a good place to do Mano-e-Mano if you haven't already. Just don't use your batarang on one when they're chasing you.)
After you finish both off you'll be awarded with the achievement.
Story related and cannot be missed.