For those of you who can't get together four people to do this achievement, I'll make it a little easier for you. This is a video that will hum the vocal part for 100% on easy. You'll still need a person on drums, bass, and guitar, preferably all on easy to avoid mistakes. You'll need to have your computer volume turned up, and Mic Sensitivity too if need be, and just set up the mic close to a speaker.
When we did this, the guitarist was the one starting the video. On easy, the first guitar note is yellow, so the guitarist held down the yellow fret and hit play 2.5 bars before the first guitar note to get the video synced properly, and then strummed the first note and started the song. (Talking about the bars on the note highway that keep the time of the notes.) After that, all you have to worry about is the three human players playing their parts correctly. It took us a lot of tries, and none of the retries were due to the vocals messing up (or even being synced improperly).
Hope this helps. Any questions, just ask!