Like Andreka0 said, you need to defeat Harley Quinn and her thugs to earn this achievement. However, some people may have a problem with this fight, due to the respawning of the thugs with their guns.
This is exactly like the fight with Two-Face, where the main boss is the priority. Otherwise, if you don't beat them down quick, more thugs will spawn, no matter how many you take out. My advice is simple; make sure you clear at least three-four of them, leaving two of them to frantically search for you. When there's enough time and space, go ahead and start clobbering on Harley. If the thugs get too close to your position, just use a smoke bomb or hit a nearby fire extingushier with your trusty shruiken and it will stop them in their tracks for a short time. Keep hitting Harley until she's out of the picture, then work on the rest of the thugs afterwards.
Be careful though; I had to start over at least one time because a hostage taker was able to spot me somehow, even when I was behind them, and shot the hostage, so make sure you deal with the hostage taker first before taking care of the rest.
You will get the achievement after the cutscene ends and shows Harley hanging. Have fun and good luck!