This achievement will unlock a minimum of two other achievements on the way if you follow what I say.
Start a Manager Mode career, make sure every game has the difficulty setting of Semi-professional and that you save after every game. If you draw or lose a game, reset your XBOX 360. After 50 games, you will unlock the “Undefeated” achievements. After 60 games you will unlock the “Winning Streak” achievement, and once the career has finished and you’re unbeaten, you will unlock the “Undefeated Manager” achievement.