This achievement glitched on me after I got promoted from Division 10 to Division 9. I went to my previous game history and in my case I had played a game against a friend. I clicked on that match using the A-button and the achievement popped with no reason whatsoever since I had 7 points and 5 games remaining on my Season. I think the trigger was because my friend unlocked this achievement and therefore unlocked on my profile, but I can't really tell this for sure. It may be a glitch, but it might be worth trying this if you struggle to win a cup.
My exact method:
- Select Seasons from the 'Play'-menu
- Scroll down to 'Current Season' and select it with the A-button
- Now you should see your previous matches, advance on one with the A-button
- You will see his/her history and if they promoted and won the season cup, the achievement should unlock