This achievement requires you to win ONE cup match. However you can only play a cup game when the cup window is open.
Going to Xbox Live -> Head to Head Seasons will show if the cup window is open or how long until it next opens.
When open choose "Play cup match" and you will be put into the round of 16. You can try as many times as you want before the window closes - so don't worry if you lose a match.
As with most FIFA achievements you must play the full match and it will pop when you reach the "match end" screen.