Alright. The previous person wasn't clear enough, so I will try and say what he means.
Go into Quickplay -> Score Duel/Tug of War -> Player Match. Find somebody, and let them beat you. When the match ends, you will still be in the party with him. The person that joined the game (you or the complete stranger you're playing with) will select the song. Beat them on the song and the achievement will unlock.
You can ask a friend to boost, or somebody else. They'll probably say no, but if they accept, then there's nothing really to explain. Just follow my guidelines above and there will be no confusion.