Ok i'll warn you now, this is going to be a grind fest...!!
When you get to level 34, the bonus round (in chapter 5), just keep repeating it over and over with each song that isn't levelled up to 7. It takes about a minute to complete the level and it doesn't take too long to level up a song, if you can tolerate the repetative nature... Personally, i completed the game first and then went back to the level and went through each song that needed leveling up. Took me about 45 minutes to get the achievement. Hope this helps you out??
*If you start a song from level 1, it'll take a max of 6 repeats to get it to level 7*