The description of this achievement is a bit misleading, you do not need to perform an Alley Oop yourself, meaning that this can be achieved while playing through the "story" mode. Just play against a team with atleast 2 opposing players and if you let them have the ball, eventually they should complete an alley oop between each other and the achievement should pop.
Note: this could take several games to get, it is much easier to get if you have a friend to play with, but if you do not, or you do not want to wait for someone, you can get this while playing solo easily enough.