I would never rely on Aimed 'Head' to do anything. My aimed shot now is mastered and I can almost never rely on a headshot doing anything more than damage. I can, however, rely on torso to knock them back just about every time I hit them with it, but then again I am not sure on the lag they receive for that. I have heard it is similar to trip lag, so who knows. And I usually use sword/shield or ranger spear for PK as serpentstrike is usually a very good option.

However, for longer fights, with a bow, if you can draw it out and get your aimed shot at their legs and arms to weaken them, that helps as well.