Right after you fight Poison Ivy you go confront the Joker. You'll go into a hallway with a bunch of thugs on either side wearing party hats and cheering for you. All you need to do is start a fight and defeat them all. Careful because this achievement is missable and if you go past them without fighting them then you'll have to play the entire game all over again.
The fight can be pretty tough, especially on Hard, because the room is extremely small and narrow. If you put explosive gel around the room before you start the fight you can knock everyone down with the explosions. This should give you enough time to take out three or four thugs with ground takedowns before everyone recovers and starts to attack you. I also noticed that the explosions cause some of the thugs to glitch and keep cheering even though you're beating up all their friends.