Note that it says 55 points *IN NEW YORK*. That is, the game has to be the LA Lakers against the Knicks, with the Knicks being the home team. I did this the first time with the Lakers as the home team and obviously it didn't unlock.
As with pretty well all achievements in this game, you are free to do it on any difficulty and adjust all sliders in your favour.
Personally, I found Kobe to be the best dunker, so you might want to do the "Sprite's Dynamic 10" while working towards this achievement.