Full Auto

This Achievement is unlocked by accumulating a total of 250 kills across all your races. This is likely to come from playing through the Career mode, so just keep firing on your opponents (and laying mines or grenades behind you) as you race and unlock this one in time.

You can also rack up a ton of kills in the Rampage series events. It should be easy to get upwards of 40-50 kills in a single race.