To get this achievement you need a total of 66 online kills across all modes. The best way we found was to set up an online race since that gives you the biggest time limit. Fox and Hounds is the best for getting the 30 online wins but doesn't really work for kills since the tag keeps changing every time you hit another car. However, using the following method will help you to rack up the kills very quickly.
Have the player getting the kills pick the Cadillac, and the other player something fragile such as the rocket car or buggy. Player 2 parks with their nose facing a wall, then player 1 drives into them from a medium distance. This results in a kill almost every time, and player 2 will respawn right next to you making it even easier. The below clip shows just how quickly it will go by using this method
Sorry, this Game Clip has been removed