Real Soccer League, or RS League, is a game mode in which you play as many as 177 games against different opponents. This mode's leaderboard is the only way in the game to compete with your Xbox Live friends. Your objective is to score goals and win as many games as possible. You need to reach a score of 200 to unlock this achievement. You earn 1 point for every goal you score, as well as 5 points for a win, 3 points for a draw. You earn no points (and do not lose any points) for losing games. This means you can reach 200 with just 34 wins or with 67 scoreless ties. Not that hard considering there is 177 games. Using Chelsea it only took me 39 games played to get to 200 points.
I suggest you save this league play for last, as this will be the most difficult achievement because you cannot set the difficulty and many of the opponents you face will be challenging. Depending on your skill, you may need to play nearly all of the 177 games to get to 200 points, though that is unlikely even for the worst of players. This achievement is technically missable, so it is advised that if a game is about to end, you did not score any goals and are losing (basically, you aren't going to get any points towards the achievement), quit out as to not waste the game.