This is for people that are unable to find a co-op partner to complete this achievement. The kinect can be fooled by some objects into believing there is another player in the room with you (this has worked for some other kinect games also). I used a rolling computer/office chair to complete three worlds (my son helped with the others). The way I would do it is to stand behind the chair with my chin resting on top of the top cushion (so it looked like the chair was my body) I would wave my arms (from behind the chair) until it sensed me, and also would do this to select which arm (left or right), and pitch for the chair when prompted. Once that was done I would quickly step to the side, and then set the game options up for myself. It was definitely a little "fiddley" at times getting it set up, but once it was done it would continue to sense the chair, and I wouldn't have any issues until I had to re-setup for the next world.