I've only managed clearing out about half the Njerp territory before I have so much wealth that it's no longer worth it to me to keep playing the character. Mostly, I'd live off the spoils while my wounds healed and then whittle another settlement down until it was manageable to go in and clear those remaining in one fell swoop since I could never persuade a companion (other than my livestock and dogs) to accompany me when I really needed them. With my last attempt at this, I was razing villages and using the building materials to set up my own city that was centrally located.
This time around, I'm just playing a farmer/fisherman/gatherer and dealing with wandering warriors as they turn up. It's actually more fun this way since I have to plan ahead and get by on the meager resources at hand. I think my little dog might be getting tired of eating lake-reed flatbread, though. He doesn't complain, but there's a certain look in his eye.