in addition to the expert solo career guides above, you can also one-man band this achievement in BAND CAREER and get both achievements at the same time: Nothing Else Matters and Metallica.
**BE SURE before you start that you check/change the profile you want to attain the achievement is the band leader by going to the preferences screen where you select an instrument, difficulty, etc. then press X (blue button). you should see a coiled blue snake icon above your name indicating that you're the band leader. unfortunately, only one profile, the band leader, can obtain band career achievements.**
Play an instrument on whatever difficulty you’re comfortable with and add beginner's vocals using the mic trick (**see below**). beginner’s vocals will produce plenty of star power to boost your score and easily attain 5 stars per song.
**MIC TRICK: as an alternative to a fan, if you have the right setup, it also works to put the singer's mic into your headphones earpiece (i have full turtlebeach headphones) with the volume up to max and have vocals set to beginner, then turn on your TV sound so you can play along.**
play and obtain all 5 stars in 15 songs until you unlock the ice cave venue (72 stars total). then switch both instruments to Expert and play the final song "The Thing That Should Not Be.” achievements pop up after playing the final song - no need to go and complete all 50+ songs. The final song is easy enough to sing and play at the same time. Expert Vocals and Bass worked for me.