Simply do what the achievement says, and beat the hidden boss following the Classic Campaign with a team from the 6 NBA divisions. This requires you to play a total of 222 games, which is a lot more than most people are willing to play.
So, the way that I've worked on this achievement is as follows. Find something in your room that is fairly heavy (I've used Guitar Hero drums and a coffee table mainly). When you start the campaign pick a team who's starting SECOND player has a good overall block, rebound, and 2-pt shooting. Make sure the difficulty is set to Easy.
Now what you want to do is put your controller under the heavy object so that the "X" button is pressed down constantly. Start the game, and your player will basically jump around the court the whole game and shoot full-court shots off of inbounds plays. This may sound like it won't work, but throughout the first 4.5 campaigns I have played I've only lost one game. So basically all you have to do is hit "A" a few times at halftime and to start the next game and your AI partner will play and win for you, leaving you to do homework or whatever else you want.