they are nigh unkillable unless you are a dwarf and lucky. the real problem in killing them is their nigh invincibility. they will dodge nearly everything you throw. however, a martial trance nearly removes this obstacle. i murdered at least 10 of them one night. as soon as they sprung on me, i hit a martial trance. walked through them like they weren't there with my axe.
Are you playing "clear" DF? I'm asking because they are a lot easier to deal with in genesis mod (at least from my pov).
The main problem in "clear" version is that, while weapons are more powerful, the tissues are a lot more "pain immue"/spill a lot less blood. In order to cause somebody to pass out from pain you have to basically break/cut out basically almost every part of the body, including internal organs. Also, blood loss is very small even from piercing heart, lung, or brain; so small that the creature can "heal" the bleeding during the battle. (It basically never happened for me that somebody died due to blood loss before I killed them). This, coupled with usually crazy high dodging makes boogeyman, like cikulisu said, almost unkillable - even occasional lucky shot doesn't seem to bother them at all, even if it is bone crush of internal organ pierce.
On the contrary, for example, in genesis cutting off one arm, crushing bone from one leg and cutting off one finger from the other arm was enough to make boogeyman pass out from pain & die to blood loss. Also, blood loss and the amount of pain have some crazy high values for key internal organs, so a piercing strike to a heart (for example) means basically "game over" for you (or your enemy), unlike the "clear" version, where a pierce to a heart doesn't seem to have any big effect on effectiveness of a enemy.
What are you smoking? Cause I want some. First of all, the term is "vanilla", not "clear". Secondly, the raws say otherwise, because genesis has lower values in PAIN_RECEPTORS for skin, fat, and muscle. That means that they are more "Pain Immune".
I can also kick someones hands in or take out their teeth to make them pass out, its not hard. Blood loss from piercing the heart is near 100% chance of death, and if you tear open a lung they might die from that, and definitely from both of them. The brain is also an instant-kill from what I have seen, and creatures cannot heal during battle, or if they do its very slow. Bogeymen aren't a problem for people who have better characters, and its not hard to pin a bogey down and kill it. Just carry a peasant with you if you aren't good enough to fight them yet.
Now, about your points about genesis about bogeymen being easy and passing out from pain, guess what? Those creatures haven't been changed in the raws, they are the same as the "vanilla" version. Atleast make sure you know what your talking about bro.