Once you reach the final chapter in Act 2 (Last Stand), you must fight off a group of Locusts. At the end, an Emergence Hole will appear with five Boomers inside. To kill them all with a single shot, get into cover and wait for a small metal sheet to fall into the hole. Behind the sheet is a gas tank that, when shot, will trigger a reaction in which a giant fuel tank rolls down over the gas tank, then fall and kill all of the Boomers in one shot. After doing this, Cole says "Yeah, burn biatches!". Note: This will not earn you the Clusterluck achievement if repeated ten times.