This is very easy achievement to get. Simply kill any enemy creature that you encounter while playing. All types of enemies will count towards this achievement.
Eventually you will get this but if you want to speed things up a bit, try casual difficulty. The enemies will drop like flies and you will sweep areas a lot quicker. And don't worry if you wanna play on hardcore and you will die all the sudden. The stats for this achievement will not reset in death because this achievement is cumulative across all the playthroughs !
Happy hunting !
*EDIT* IR A V IE N informed as follow in the comments; "The easiest place to get this is, I believe, the Orichalum Mines. There are hundreds and hundreds of spiders there. Just set your difficulty to casual and you can easily get this achievement in 1 day."