Go to the "Main Menu">"Game Modes">"Be A Pro", then select the Boston Celtics and Los Angeles Lakers as the two teams. Now hit LB or RB to make it a Rewind Game, make sure the date says 06/05/2008. Now play as the Los Angeles Lakers as you will need to keep Paul Pierce(Boston Celtics) from scoring over 10 points. Don't forget to set the Quarter Length to 10 Minutes as you will not be able to get the achievement without doing that. The Difficulty can be set to anything. Now once in the game go into the Pause Menu and go to Settings>Rules and turn off Shot Clock and 8 second violation.
Now when you have the ball just hold on to it by your opponent's basket and let the clock run down. You do NOT have to win the game to get this achievement. All you have to do is keep Paul Pierce's point total under 10. So just hold onto the ball every time you have it running down the clock to 0. Once the game is over if you have successfully kept Paul Pierce's point total under 10 the "Cool Off" achievement will unlock.