After you finish the main story as Batman, this final section of Catwoman's story takes place. When you get to the Two-Face fight, the enemies will continually respawn until you take him out so just wait for him to be alone up top, knock him over and then just keep hitting him.
After that, the enemies with your loot will have a glowing diamond on them when you use thief vision. They can be found in the Bowery and Park Row areas of the city. Simply knock them out in any fashion to retrieve it and once you fill the red bar under your health the achievement will unlock.
If you didn't have the Catwoman DLC while playing the main story, you can do them at any time from a selection on the main menu of the game.