JUST DANCE 2015

When you go to play a song and are at the point where you select a character, you will see a button in the top right corner for you to add challengers. They will dance along with you and at the end you will be ranked among them. As long as you beat one, you can click the button on the screen to send a challenge to that person to beat your score. This will unlock the achievement.

You need to send 100 challenges, so keep adding challengers and sending out challenges after each song until you unlock this:


and