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 -> Speed Bag
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 punch a few times in correspondence to the speedometer. Make sure that the proper player has the higher score, and after 3 wins, the achievement will pop.