Kill 2 enemies with the same shot using the Hyper-Cannon.

When you have it, you can aim down the sights by holding 'LT' and the crosshair will fill up in a circle, when this has filled up, or 'charged' you can fire the weapon with 'RT' for a very powerful shot that will travel through enemies.

For this achievement you will need to find a large enough group of enemies, progress through the GreenHaven Ark to find a large group of mutants that you need to kill, then have your weapon charge up and try to get 2 or more enemies in your line of sight and then fire. Where you are attacked by the mutants in this location, 3 of them will stand above where they spawn and throw projectiles at you, these will not move, giving you a perfect opportunity to line up a shot on them.