Hi all,
On your map, hovering over different areas of Gotham will show where there is a League of Assassin's presence. The symbol is a circle with some diagonal lines (you can view the map legend with
). I just hopped around some minor crimes (white triangles) in these areas and popped it pretty quickly after finishing the story. As a bonus, you can work towards the Man-Bat and 250 crimes achievement while doing these.
Note: I'm sure there is a way to grind out these guys in the heroic showdown floors, but since it takes more time to get to those higher floors than it does to just do a couple of crimes, I would advise against that method.