In act 2 "the city" there will be plenty of humans to kill called punks. You will run into some during a couple quest lines and some will be hiding. They will most likely be carrying a pistol so be careful around them. Grab one of their guns and aim for their heads. It usually takes 1-2 shots to the head to drop them.
If you have completed act 2 and cannot find any humans, the next act "the jungle" has plenty of humans to kill with assault rifles and shotguns
After the 50th human, your achievement will unlock.