By master, it means earn five stars on all songs in the Flash4wrd Crew Challenge.
The songs are (Number indicates difficulty level stated in game):
Get Ur Freak On (7) (as the final, unlocked after 16 stars are earned)
Rude Boy (3)
Hot Stuff (3)
Right Thurr (3)
Whip My Hair (5)
The songs can be broken down from within the song selection in the Crew's setlist, so you can have a good practice/warm up. The higher difficulty songs are a little hectic and overwhelming to sight read, but break it down is a massive help.
This IS stackable, so pop this and you'll also get the easy and normal achievements if you pop this.