You just need to simply win an online game. It can be against any number of players, however playing against only one other player will give you the best odds of winning. From the Main Menu, you can either choose Play Online, which allows you to select a custom match based on some criteria, or Quick Match, which will pair you in any available game. The online community for this game at any given moment will likely be small, so you may need to boost this. See THIS thread for others looking to boost as well.
NOTE: This achievement is stackable with "Best of the Best." Winning a game where all seats have been taken will also unlock this achievement.