Use two controllers with 2 minute halves. Make sure to turn off the shot clock and make sure you get the ball at the end of the game. Score, don't score, it's up to you but it's easier when it's 0-0. With 2 secs left have your best shooter go for a three and you should hit it. If not go into overtime and repeat. 9 times out of 10 you should hit the buzzer beater and the achievement will unlock immediately not after the game.