This achievement is a little glitchy and will almost certainly unlock early.
Every time you play a song you will get progress towards the next rank by earning stars or gaining other bonuses like gold starring a song (bonuses are listed below). In theory this will not lock before you reach level 40, but in practice, this unlocks somewhere between level 20 and 30. In performance mode, you can simply go through songs one at a time, constantly picking a new outfit for each song and keeping track of what you have and have not used. For me this song unlocked at level 23 when I used Miss Aubrey's classic outfit, though others have reported differing levels in the same area.
You will gain a level and unlock a new outfit when you gain a certain number of stars. In addition to the stars you gain in the song you will also get stars for doing the following:
Gold starring a song (+1)
Playing in Fitness mode (+1)
Playing a DLC or imported song (+1)
Playing a new song (+1)
Playing a song with a character on their birthday (+2)
Playing a song in a setlist (+1)
Playing a song on Medium (+1) or Hard (+2)
Playing a more complicated song (+1 or +2 depending on difficulty level)
There is a +5 Random bonus that will come up from time to time as well.