You can either do this online or with a friend. Make sure you're in a Shootout or Gang shootout game mode and achieve five headshots within the match. While aiming at other players, lock on to them with the push up on the to aim for their head. If you don't already do this you should as it's an instant kill with most rifles.
If you take cover a lot and stay still the other players on the map won't be able to see your position. This will help you pick off the odd person running around on their own trying to snipe others.
See "Call it a Comeback!" for boosting information.