This should come naturally during your game play. Make sure to kill all the enemies you come across.
If you are missing a number of kills 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. You can also do "Stewie-nator" here.