This will come with natural progression. You can start this as early as level 1. When it comes to dealing with the enemies, aim for their heads. You will know when you get a headshot because you will receive double the money you would for just get a normal bodyshot kill. If you are still missing this after you have completed the game or want to grind this out replay level 7 "Santa's Sweatshop". Enter the sweatshop, flip the switch to make the elevator active, ride the elevator up, and on the second assembly line there will be an infinite spawning of suicide elfs.