Easiest and guaranteed way:
After you hear Joker supposedely talking to the weekly comedy magazine you move right until the screen adjusts and the first guys start showing up on the right corner.
What you need to do is to position Batman in that corner, with no guys behind him
. To do that you'll probably have to jump over the last guy to the right. Once there's no one behind (to the right) Batman, you must start doing the beatdowns (
) on the first guy to his left. That way, every goon waits their turn to be beaten. However, every once in a while, one of them will try to move behind you. Once they do that, even if they are not totally aligned behind you, you can still jump over and position yourself as the first one on the right again and continue the beatdowns.
If you mess up, don't finish the last guy and simply reload the checkpoint.
Once the combo meter goes a little over 100 the achievement will pop.
If this solution helps you in any way, you're welcome to give it a thumbs up and leave a comment.