As the description says, you get this for solving the Riddler's last puzzle room. This puzzle room involves getting one or two lasers into a gap by moving Riddler pressure pads around.
You need the Remote Electric Charge gun for this. You can get this gadget by going to the GCPD evidence room and smashing the Batman display case.
will pull the pressure pads towards the generator, and
will push them away.
Catwoman is extremely important for this puzzle. Something I did not realize was she can grapple to the ceiling and climb into the area with the pressure pads. If you stand on a pressure pad, it will not move, so this is very helpful for getting gaps in the middle of the room.
Every time you get the lasers into the gaps, giant saws will come across the floor. Make sure you switch to Catwoman and grapple to the ceiling as quickly as possible, using
. She can only grapple once she is directly under it, so run in the direction of the door you came in and look at the ceiling.
After you complete the puzzle three times, the key will open up. Collect it, and get 10 Gamerscore richer.
If there's any way I can improve this, let me know. Thanks and good luck!
As per Say Goodbye Sir's request, here is a video showing how to do this.
(All credit goes to the creator of this video, PowerPyx)