In order to get this achievement, you have to play through the Riddler's story (Chapter 1) as The Riddler or Two-face.
Near the end of the last episode in the chapter you will be near a vault and you have to build a giant robot with a minigun, which in order to get the parts in front of you takes a bit of co-operation between Two-face and The Riddler and whoever builds it gets the achievement.
It will pop up when you finish building the robot. Good luck :)