Killer Kodes count for this achievement.
Pause the game and go to Killer Kodes in the menu.
Enter any 13 of the following codes, and both this and the one for 13 killers will unlock immediately)
thescooper (added 11/18)
(Edit 2/18, from Steam Community Forums post by Vongalin)
(contributed by JiglifiedmQfQ) Unverified as I've uninstalled it from Xbox, so if anybody could update in the comments about them I'd appreciate it.
For some reason this doesn't unlock the achievement for getting 1 killer, but spending $1000 for that achievement and being able to use the $12000 this will save you towards gore packs will definitely save some time.