This achievement is slightly misleading.
You must win ALL matches in a single Master League season in the league, cup and Uefa Champions League.
I recommend setting the difficulty to 'Amateur' for the season. You can then aim to get the following achievements along the way:
Possession Play
Awarded for achieving a possession rate of 60% or higher in your 10 most recent matches.
Hat-trick Hero
Awarded for scoring 5 hat-tricks.
Ace Striker
Awarded for playing 20 matches, scoring in recent 5, averaging 2 goals per match.
Gentleman
Awarded for an average foul rate of less than 1 per match in your 10 most recent matches.