First of all, there is no online mode, so this has to be obtained locally.
If you have four people that you can gather to work on this, great! Simply pick any song and any difficulties that you want based on what works best for your group. But this guide is written more to help those that can't get four people together...
If you are like me, getting four Rock Band players together locally might be hard. I was able to get this achievement with a total of two people playing the four instruments! Here is how I obtained this achievement with only two people:
Put all four instruments on the "super easy" difficulty. I cranked up the sound and put the mic in front of a speaker. 100% vocals. My friend laid both guitars on the floor and strummed away like mad since you don't need to use the fret buttons on "super easy". A well timed strum is all you need. As long as you hit every note on "super easy", you'll be fine and you don't have to worry about the non-note strums you make. Bass/Guitar 100%. For the drums, I turned the foot pedal to automatically hit so I only had to match the beat to the drums. I didn't even have to hit the right note due to the "super easy" difficulty, as long as I hit something to the beat. You can even hit the "A" button on the drums if you need to. Drums 100%.
FYI- we unlocked this on the dlc song "Charlene (I'm Right Behind You)" as it's one of the shortest songs in Rock Band.
I never thought the two of us would get this achievement, so I wanted to post this to prove that it's not only possible, but it's actually rather easy. This may be a basic strategy, but I hope it works for those who can only get one or two extra people together for a local session.