Preparation - Reunion
Upon reaching the trailer park, you should notice some guys playing baseball. You'll want to have one of the players start batting and the other to start pitching. You'll need to hit a "home run" which should register around 180 points. Just aim for the middle of the screen and work on your timing. The characters will mention a home run if you had enough distance and the achievement will unlock for the batter. Switch places for the other person to grab their achievement.
This achievement can be seen in the video guide at 10:02:NOTE:
Achievements are only available to owners of the game. Players using the "Free Trial" friend pass will NOT unlock achievements.