For this achievement, you will need to get 100% on both sections in every song in Break It Down Mode. This means the first part where they make you play each move individually, as well as the recap which combines all moves. You do NOT need a diamond in the first part, as a green check works just as well. Basically at the end of the song, the total score must be 100%.
All songs can be done on easy, and the achievement WILL unlock. However, there is a weird piece to this. You must complete the 100% on the hardest difficulty that you have ever played a song on. When I got it, I tried easy on all songs before playing any other difficulty, with the exception of Drop It Like Its Hot, which I made the mistake of attempting on Hard at the very beginning. When I got 100% on everything on easy, the achievement did not pop. However, remembering that I had tried Drop It on Hard, I went and tried to see if it would pop upon getting the 100% on that song. As soon as I hit my 100%, on hard, the achievement popped.
So in short, play through this mode on easy, securing your 100% on each song, before attempting any other difficulty in this mode. You CAN play Perform It! mode on any difficulty you want and it will have no effect on this achievement, so don't panic if you've done that.
Lastly, I know that many of you have probably attempted this mode on hard for a number of songs. What I would recommend in this case is to either delete your save for Dance Central completely and start this mode from scratch, or (the better option as you won't lose your work) you could start a new save on a separate storage device. Then simply go through Break It Down on EASY for every song, get your 50g, and be done with it!!
Hope this helps, and I apologize for being so long-winded, but I wanted to be as clear as possible. Leave some feedback if you'd like, and be sure to vote positive if this helps.