First set the difficulty to "Pro" before you start the game. Then when you are pitching, get the FIRST THREE batters out. "Retire" means get them out. A strikeout, popout, or however you do it. If one of the first three batters reach base safely, then you can't get the achievement that inning. However, this achievement can be earned in any inning.
NOTE: NOT 100% SURE, BUT I THINK THIS CAN NOT BE DONE WITH A SECOND CONTROLLER, CONFIRMATION WOULD BE APPRECIATED