Throughout your PGA Tour season, you will play in various championships. The FedEx St Jude classic is NOT what unlocks this achievement. At the end of your regular season, assuming you played well enough, you will get notified that you made the FedEx cup playoffs. Assuming you have a high enough standing, winning the last tournament grants you 1000 cup points, and a victory there would likely keep you in first place for the cup standings, earning you this achievement.
Difficulty does not affect achievements in this game, I was able to earn everything on easy with beginner clubs. You can play one round tournaments to make this go by quite fast. Just keep in mind there will be sponsor challenges that require two rounds. You can adjust the rounds per tournament in the settings menu.