After playing one full season at Division 5 in the Spanish League and - as a word of advice for this - my suggestion would be to get this achievement in the first season you play.
Once you start moving up into higher divisions, it can become more difficult and time consuming to achieve this without multiple restarts of the game.
One other vital point in this achievement is that you can't draw any games in the season. In my Division 4 Spanish season, I won 34 of 38 games, drew 4, but did not get the achievement.
Ultimately, I got this achievement in a Division 3 season, winning all games, including Cup ties.