The top of a goats head is a battering ram, so especially thick. As mentioned, small target so the shot would need to be placed well.
One of the things not often considered when looking for "the most humane way", is the comfort of the person doing the slaughter.
Ex. I have seen people slaughter chickens in a cone with a few swift slices to the neck, and been told this is the most humane way to do it. I have also seen folks behead them with a hatchet, and been told that is not so humane. However, I have seen people who are not comfortable doing with it the cone and slice method who end up not making good cuts, prolonging the process, and putting the chicken through a decidedly less than humane slaughter (not to mention really rattling the person doing the work). If they were more comfortable swinging the hatchet, it would have gone much better for both the person and the chicken.
I would think about your goats similarly. I don't know what your experience is with slaughter in general. If limited, start by doing it the way you can get it done. Over time you will learn what works best for you and the animals.