Traps are virtually useless against Njerps.
I've conducted an experiment where I set up a number of pit traps (with spikes), a bear trap, and, I think, a wolf trap and lured Njerps into them. The results were quite disappointing, as they just crawled out of the traps with minor injuries (although the bear trap managed to knock the victim unconscious briefly). It also required quite a bit of manovering to the the buggers to get into the traps in the first place.
Trap lines are detected by humans, so they won't walk into those. If you're going to waste your time using traps you should make a mine field instead.
Trap lines are useless against Njerps. I had one come to my homestead and disable about a dozen pit traps before I finally killed him. At the same homestead I earlier had a vagabond visitor, and that bugger cut down a fair number of trees, including ones forming part of the border trap line. Once the trees were gone the trap line was no longer visible to him, and he stepped into one of the traps, broke his leg, and kept lying there for about two months, all the while insisting everything was fine.
Yes, you can burn down buildings (I tried once, but the Njerps woke up before I'd finished placing the branches I indended to light the fire with. I have, however, accidentally set fire to my own homestead more than once by hitting the wrong key).
I had a character I ended by raiding Njerp territory. He died after having depopulated up towards a dozen villages. The main strategy was to fire arrows at enemies coming rushing while backing off, and this usually succeeded in killing most of them off one at a time before they reached him (he moved fairly quickly). Occasionally he made a mistake and enemies caught up, but they tended to be injured and exhausted at that point, and not really able to do much against the thick layers of armor he wore.
It ended in one of these cases where a couple of lucky enemy hits injured him severely, and eventually he couldn't move, and was slowly beaten to death.