To get this achievement, you must first register a rival that has completed the LEADER BOARD arcade route, as you cannot get the achievement by beating their times in the other routes. The easiest way to do this is to go to Xbox Live Leader Boards and press LB twice to switch over to Arcade. Then scroll down the leader boards until you find a time you are sure you can beat. Once you have found the player, register them as a rival by pressing Y to view their license and then START to register as a rival. Then all you have to do is go to Arcade, leader board route and beat their time.