Instructions
Select "Options" on the "NBA 2K11" main menu, followed by "My NBA Settings" and "Game Sliders."
Set the "3 Pt. Success" slider to 100. This guarantees a large majority of your three-pointers are successful.
Return to the "My NBA Settings" menu and select the "Gameplay" option.
Set the difficulty to "Rookie." This ensures the computer does not attempt to block your shots as often and does not do a good job when they do attempt to block shots.
Return to the "My NBA Settings" menu and select "NBA Rules."
Select "Off" next to "Shot Clock." Turning off the shot clock gives you unlimited time to make a shot.
Start a game and select your team. While any team is suitable for the achievement/trophy with these slider settings, selecting a team with a high rating -- such as the L.A. Lakers or the Miami Heat -- allows you to get it that much quicker due to the high statistics of the individual players.
Shoot the ball outside of the three-point line each time your players have the ball. When you have made 15 three-pointers, the achievement/trophy appears on the screen. With these settings, you should make 15 three-pointers by the halfway mark of the game.