Here is how I managed to unlock this with a SG in MyCareer. I maxed out my dunk and vertical jump skills. With these two maxed, I would actively try to drive to the basket and ran out on fast breaks which usually got me 1-3 dunks a game. The next time I played the Miami Heat (the game was in Miami. I don't know if this matters or not,) there was a cutscene before the game with LeBron where he mentioned that I dunked a lot and taught my player a dunk move that he likes to use on fast breaks. After the cutscene, the achievement popped.