To make this easier enable Auto Aim from the options (Press - Help & Options - Gameplay - Auto Aim ON
Soul killing means that the Deado releases a ghost/soul as they die opposed to normal deaths. Soul kills of a Deado occur from headshots
so aim for the head! (Aim down sights is
This takes a lot longer than you'd expect especially if you primarily use the shotguns but will come long before you complete the game, the Revolver
is a good gun for this.
Here's a short video showing you what expect with a Soul Kill