You will get this over the normal course of the game if you enjoy the online play. If you want to speed things up play sniper rifle only games on smaller maps, and you will net this in no time.
Considering how old this game is now, you will probably have to make a boosting session for this, very easy to boost though.
Simply make a sniper rifle only elimination match on any small map and simply take turns headshotting eachother.
Also, make sure that it is a ranked match.