This challenge simply requires you to stealthily subdue people. Similar to Piano Man, you can pick any easy level that you are comfortable with and just start subduing people.
One of the easiest places is on Terminus - Upper Floors. Start the level, take a left and another left so that you are in a hallway. You should see a man guarding a window ahead. follow the hallway until you come to the cart with the radio. Turn around and follow the hallway out to the main area where you will find a bathroom on the left.
Take the radio to the end of the row of sinks and drop it on the floor by pulling the shoot button. Turn the radio on and then stand in the open stall. When someone comes in, let them turn off the radio and then subdue them. Drag them back into the stall you were standing in and then turn the radio on again. Repeat this process until it unlocks.
Just remember to let them turn off the radio because if you don't, as soon as they are out of the picture, another guard will be sent to turn off the radio and may stumble upon you cleaning up the body.
