UFC Personal Trainer

This achievement can be tough as nails to get on your own. However, if you have cardboard cut-out of a person that the kinect sensor considers legitimate, you can attempt to do it that way. However, the achievement is actually really, really easy and takes maybe six minutes to unlock - if you have someone to do it with.

Main Menu -> Multiplayer -> Side-by-Side -> Tire Flip
Select Beginner. (Higher difficulties will require that you play it for a longer period of time.)
Then, after it starts, have the signed in player squat and then lift his/her arms up to imitate doing a tire flip. Then, you can wait out the timer - making sure the proper player has the higher score.
The achievement will unlock after the third activity is completed.