You need to play at least 35 games and end up with the best record in the NBA for this achievement. You should also play the deciding game to lock up home court.
Play the first 35 games (can be done with 1 minute quarters and sliders on)
Sim the next 25 games
(If you had a record close to 50-10 after the first 60 games you will want to monitor other teams and make sure 1-2 teams are still close to you in record. Keep an eye on games left in the season and your wins to theirs)
Make a duplicate save file and turn auto save off
Sim the remaining games until you are one game away from clinching the best record in the NBA
(EX. You are 60-17 with 5 games left. Next closest team to your record is 56-22 with 4 games left)
Play and win the next game. The achievement will unlock when you are back in the menu.