Two of the achievements in the game need you to get 500 kills and 1000 kills. Now trying to get to 1000 kills by simply racing normally will take forever to do, luckily there is a quicker way. Simply go into the rampage mission in the Training series and keep repeating the mission until you have got over 1000 kills. You should get 60 kills in every few minutes, so it doesn't take long for it to all add up and get the achievement unlocked easily.