To unlock this achievement you need to uppercut an opponent that is on the receiving end of an ally up pass. An ally up pass is were you hold
to pass the ball to a teammate near the basket. A successful ally up will trigger a dunk.
So, by far the easiest way to unlock is to use 4 controllers and set up an arcade game. At the start, use your guest accounts to take the ball and stand in the near left corner. Take the other guest player and stand him near your basket. Move your player within uppercut distance of the player near your basket. Hold
on the controller of the player with the ball and time your uppercut (
+
) with your player to hit the receiver just as he gets the ball. It make take a few attempts to get the timing down.
If you don't have 4 controllers it's easy enough to do in the story. Uppercut any opponent in your half of the court, near the basket, and you will unlock eventually. Good luck.