This can be unlocked on the level Jasper & Rose. Start the level, scan the door and get rid of the first couple of guards. Go up the stairs to the top floor.
On the top floor, simply pull out your gun and kill all of the guards in less than 30 seconds. This should be quite straightforward if you are used to the corner gun as there are only 3 guards and 2 of them stand together at the start. However, 30 seconds is a long time for 3 guards so this should be quite easy regardless.