This will come naturally over the course of play. However, I appreciate sometimes this achievement comes up in competitions on this site and you need a quick way to crack this out.
The quickest way of ramping up kills is level 2-1 you can speed run through much of the level killing all the easy enemies as you go. Eventually on the left hand-side you'll see an old barn/farmhouse. Run inside this farmhouse and interact with the altar and then run back outside and then back inside again. You'll then be greeted with over 100 spiders you can kill in a very short amount of time. A few runs of this level and the levels to get to here and you'll be done in no time.