This achievement sounds a slight bit misleading, you only need to destroy 1,000 enemies. You can replay levels as often as you like and enemies killed will add to your total. It doesn't have to be 1,000 different types of enemies (as there aren't that many). Use the Xbox One Achievement Tracker fuction to track your progress as it is very helpful. Also if you took the time to vote negatively, please take the time to explain why so I can improve this solution for your benefit.