This is the easiest League achievement out of them all.
Firstly, go to League, then choose International League and simply set it to 3 teams. This means you only have to win 4 games for 40G. Not bad eh?
Also if you have no thumbs you can set the difficulty to Beginner and choose some really rubbish teams.
Boom! achievement unlocked. 40G's.
EDIT: You can also change the settings to half of a season and only have to play 2 games. Thanks to - nathandl1994