I got this achievement after beating the game (I state this because I had previously traded players for Pros/Legendary players throughout the game, not keeping track of how many teams they were from).
Pretty simple achievement. I just did "The Boston Open" under speedball (mainly because it's a easy, short, and a 5v5 tournament). Every time you complete a match, back out and change your lineup for the first 5 people (or however many are going to play for the tournament you select) selecting them from a different team each time.
Like I said, I had already beaten the game....and after 3 "Boston Open" tournaments I got the achievement.
Just make sure to change your team after every match to get the achievement faster.