This cheevo will be awarded for scoring a platinum medal on any song in score attack mode on hard.
Pick any song you are comfortable with and in the songs menu press LB to do score attack mode.
Now select easy, medium or hard, medium for this specific cheevo.
Now for me personally, I used "Go with the flow" by Queens of the stoneage (RS1 pack required).
This song is super easy and only uses 3 chord shapes (open E + A2) (A3 + D5 which moves up to frets 5 + 7) and (E5 + A7) and on medium uses even less notes than the video shows.
To get a platinum medal you must get perfect phrases throughout the song. To makes sure you are getting perfect phrases the phrase boxes at the top of the screen will turn green, if any turn yellow then you will have to restart the song.
The video below shows the RS1 version and the 2014 version only has a subtle difference when changing chords.
This cheevo also stacks with the , hard and master version meaning if you get the master mode cheevo you will also get the medium and easy cheevos too.