Achieve a kill chain of 10 or more in any Advanced Co-Op mission.

You can do this on just about any mission where there are more than 10 enemies on the screen at once. I suggest you setup a private mission with another play on "The Herd". At the start of the level you will see the camp below, select the Bolt Action Rifle as your primary weapon and use the Rolling Block Rifle to snipe the enemies. Run across the top of the canyon and shoot the TNT box on right side of the camp with the Bolt Action Rifle. Now change to your sniper rifle and pick off enemies.

While you're doing this have your team mate if use any of the Repeaters to hurt the larger enemies. They can also shoot other enemies to weaken them for you. Carefully aim for the head and chest, you don't want to waste more than one bullet on each enemy.