This is done in level 7 "Santa's Sweatshop". When you enter Santa's Sweatshop, you will notice a picture of Vernon on the wall. Vernon will appear from around the corner and come after you. Simply kill him and the achievement will pop.
He's in Santa's Sweatshop and looks like every other big elf, but instead of wearing green he's wearing red. Honestly just shoot like mad and you'll get it without a problem(that's what I did and it worked)
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