Serious Sam 4

Very easy. You can do it at second level on tourist difficulty when you get the double shotgun and the first werebull appears. Shot his face when is very close to you. After will appear two more, do it the same and the achievement will pop.

Simple achievement to do. Whenever you have a Werebull spawn in, switch to the double barrel shotgun and try hitting them in the head to one shot them. It's much quicker to do this on Tourist (Easy) mode as you pretty much one shot them anywhere on the body. They're cumulative kills, so doesn't have to be done in quick succession.