By master, it means earn five stars on all songs in the Lu$h Crew Challenge.
The songs are (Number indicates difficulty level stated in game):
Turn Me On (2) (as the final, unlocked after 16 stars are earned)
Nothin' On You (3)
The Breaks (2)
Fire Burning (3)
Real Love (1)
Last Night (2)
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 if needed.
This IS stackable, so pop this and you'll also get the easy and medium achievements too!