Spoiler Alert: The Only Things Worth Killing After 17

SPOILER ALERT - this one is Game Master material. If you are going to play the Outskirts rather than run it, skip this post.

Past a certain level there are exactly two things in the Outskirts worth fighting: the moat warden, and the king with all his guards.

That is a feature. A frontier should run out of monsters before it runs out of trouble.