Generator Rex: Agent of Providence

Most people will unlock this naturally through play progression on your first playthrough. But if you really want to grind it out, or you somehow ran through most areas of the game without reaching 200 kills. Start up the last level as it has the highest volume of enemies to defeat, using whichever fighting skills and weapons you find easiest and play through it. As well as if you're looking to get the achievement the final level is where you'll most likely be grinding anyway. Best of luck out there!