Guitar Hero: Smash Hits

You can get this achievement very easily (along with Band Leader) if you have all 4 instruments. Just simply connect your instruments and put them all on Beginner difficulty. At this point I would also recommend that you put your mic up to something that makes noise, such as a fan. (You could also just say random things into the mic since it's on Beginner but that takes work =P)

Now choose any song (I chose Them Bones because it is relatively short) and just sit back and watch the song. I occasionally hit some notes wirth the guitars and drums just in case I failed, but honestly I don't think you can fail on Beginner. Just don't hit too many notes with the other instruments and let the mic do all of the work.

Once the song is over 2 achievements should pop. "One For All" and "Band Leader"!

If you have all the instruments you can just hook them up on beginner and play through a song to get this achievement.

Other wise you can just play in band quickplay on xbox live, but chances are you won't find a vocalist or at least one other person, so to help your chances just add a mic and play your instrument of choice.

You can find people on the forums here that may want to boost this achievement too and just add them.