My favorite song to get this achievement on is "Ring of Fire" since I like the song and it's actually the shortest song in the track pack.
just to clear things out,
- when the achievement says gtr, bass and drums it means that the song should be played with all these three instruments together, in a band, and all of you should hit the SP notes that comes at the same time on every instrument. this is for those who doesn't know what a unison bonus means.
- It doesn't matter whether you use your SP or not, so if you have your SP and get more unison bonus that's totally fine.
- Difficulty doesn't matter, so you should use whichever difficulty you feel comfortable playing on.
good luck :)