The Gunstringer

Pretty self-explanatory, but potentially a bit tricky if you lack friends. In that case, you'll have to trick the kinect into registering something else as a person. Anything roughly humanoid-shaped should do it, like a jacket on a clothes hanger or a chair, for example. Just make sure whatever you use can do something the kinect interprets as the "person" raising its hand, like lifting up one of the sleeves on a jacket.

Keep experimenting until you've gotten the second player to join the game, and then just choose any act, play through it to the end, and the achievement will unlock in the stats screen. The second player won't have to do anything, so don't worry about that. A good level for this is act 2-2, where you fight Gatorjack, since it's very short and also easy to get the Hardcore/Gold medals on for the "The Last Undead Boyscout" achievement.