Host a private coop game online. Note: You must have someone that knows what they are doing, and if you do single player the CPU will engage the Locusts. Once you have a friend in the room, work on the campaign or just load the chapter 'Imaginary Palace' on Act 4. Make sure you have a sniper rifle. When you first get inside the mansion, clear three Locusts upstairs and plug up an emergence hole downstairs. Afterward, go in the back of the house to get to the secret labs. Once there, reach a gate that you must use the wheel to open by turning it. Once it is open, stop. Your partner must not engage the Locusts-- have them just sit back and relax. You must travel down the stairs until you reach two Locusts. One will be trying to kick open the door leading to the wine cellar while the other will be just sitting back. Sit in the doorway of this small room and make sure you have your sniper rifle out. Get a headshot on the Locust while trying to kick the door open. The Locust who is sitting back will run start dancing back and forth between the door the now dead Locust was trying to kick open, and the southeast corner of the room. You may have to do this a few times until this happens.