This can be done on any level. Throwing knives are an easy weapon to get a kill with, both individually as well as consecutively, because you can reuse them after each kill. Because you can only carry 5 knives at a time, you will need to pick up any that you've used. Pick them up from the dead bodies and reuse your throwing knives. Any level that has a large amount of enemies is good. Levels like Revolution or Facility are the way to go. The achievement should unlock when you kill your final enemy.