In considering how to handle ‘fatal blows’, specifically how to cause a particular death blow, I wanted to find a way to target specific body parts. I can see cases where a particular killing blow would be useful (staking a vampire or decapitating it both come to mind).
I believe I’ve found a fairly simple way of handling this, working in two parts.
If you can apply the coup de grace action to a creature (it is helpless and you take a full round action to do so) then you can accurately hit any part you can reach. If this is sufficient to kill the creature (either directly through hit point damage or by the failed Fortitude save) you’ve landed the killing blow you wanted.