Kinect Sports Rivals

Start up the game, sign in and all that good stuff. Then once you reach the "Island" (or as it's commonly known "Menu"), swirl over to "My Game". Choose "Friends List" and then "Friends List" again. Hopefully someone there will have a character rather than a black square. Pick someone, despite having no indication if they are online or not, then choose "Invite to Game". You then have to wait for that person to accept your game invite. Once they join your game, choose one of the Target Shooting Events that has a 2-Player icon, and win.

You can also go for this achievement:

Make sure you choose the Blind Fire power-up before starting the session, as you can't change your character while you have other players connected. Once you are playing just use Blind Fire on the other player when there is a good number of targets on the screen. You should have a few chances depending on how fast your power-up recharges. Also this is a cumulative achievement so if you don't get it right away keep trying.