Let the Music Do the TalkingG 40

Gold star rating only pops up when the song is completed with 100% of the notes hit. Now, with a buddy, complete a song with 100% notes hit in a co-op song, and the cheevo pops. It works for the easy setting if you're having trouble.

EDIT: DynafireSH and mattclarkie confirmed that only the player wanting the achievement has to obtain 100% notes hit, and the buddy doesn't need to. Theoretically, you could turn on the no-fail cheat and do it by yourself without a buddy. Thanks DynafireSH and mattclarkie! (unconfirmed)