This achievement has a suprisingly high ratio for how easy it is!
First off, even though it's a co-op achievement you can get this on your own provided you have 2 controllers, set a local co-op game up and pick a song, make sure you're playing on expert and when the song starts up just strum wildly until you fail the song, upon failing you'll be asked if you want to retry, retry the song and strum out again, rinse and repeat and you'll have this achievement in no time!