Since I play many roleplaygames and also lead one as a DM I need to say here sth. about golems, undead and stuff. Most of my knowledge is about the universe of the Forgotten Realms, maybe in other fantasy universes things are being explained in another way. (The Forgotten Realms is imo a truly great designed universe (till the 2nd Edition at least) and its not wrong to copy from this universe)
First, all these creatures are magic driven. Many of them (especially golems) only exist because a magician has decided to create a monster out of stone, earth, adamantine, whatsoever. Some of them exist because of the nature of the place they are being created. Undead for example are mostly met in scenarios where many people died. May be a doomed fortress, a place where a big battle has been faught, a graveyard etc. Trolls have (lets say) genetical benefits, which allow them (and only the creature which lost the limbs) to regrow limbs and to recover wounds within seconds (they are not the only but most popular exemple).
To come to detail in killing these creatures.
Some here said skeletons cannot be killed, because they have no living part in RL. Well yes, but have you ever seen a walking skeleton with a sword and a shield hunting a dwarf with a battle axe in RL? Please, never compare non fantasy stuff with stuff only existing in fantasy worlds. You will just fail.
For golems its usually some kind of magical stone magicians use to enable a solid rock to live. destroying or departing this livemaker from the rest of the body will always lead to destroying the monster. Spears and blades won't do much (if any) harm to the golem, except for flesh golems. This is where damage resistance should come into play.
Skeletons easily lose body parts by attacking them with hammers. (Much easier than their human equivalent) Though it will not "hurt" them they will deal much less damage with no arms and hands on their body. Departing the upper body from the lower body, or the head from the body will lead to sudden death though.
Zombies are rather equivalent to skeletons, they are more vulnerable to swords than to hammers though. Spears won't do much harm either. I have never thought about if Zombies live after them being hacked to pieces, but they are rather harmless after being chipped to small pieces and therefore considered as dead. (lets not make it more complicated than it is anyway)
Well trolls can be only killed by acid or fire as many here will know. A troll has no vital point and it won't be killed if you chip of its head, putting acid or fire on current wounds will stop regeneration and therefor can kill a troll. As no acid or magic fire weapons are implemented in dwarf fortress yet I would simply add a vital point and let their wounds regenerate much faster than the ones of other humanoids. (e.g. the actually bugged invader type regeneration, as for me trolls are being killed much more easily than goblins atm)
long post, but at the end I hope I can add a few suggestions to the discussion.