Dance Central 2

Please ignore the solution that WAS here, it was incorrect

Gold Stars are not related to overall score. They are related the score of each move (i.e Flawless, Nice, Almost and Miss).

A Gold Star will be gained if you can Flawless at least 80-85% of the moves in a song and get at least Nice on the rest. It doesn't pertain to difficulty, so it can be done on easy (or so I have been told). However easier songs on easier difficulties require you to Flawless more of the moves in proportion to the whole thing.

An Almost or a Miss may be acceptable but will hurt your chances.

Gold stars appear after you get 5 stars, but the achievement will only pop once you complete the song.

In addition to what tsunamishadow wrote, one thing to note is that you can get this achievement when doing a fitness routine. Anytime you can kill 2+ birds with one stone, you might as well.

For those wondering, I got the achievement doing the 35 minute routine (I think it's called 'Intervals'). I think I got the gold star on "Bulletproof". And...I did the routine on easy.

Since I hadn't done anything related to fitness mode prior, I got 3 other achievements during the routine: completing a playlist (pops upon completion of routine), completing a fitness playlist of at least 15 minutes (pops upon completion of routine), and doing a playlist of at least 30 minutes (pop mid-routine).