I managed this after a 3 reloads. I didnt do anything really different from other boss mobs. Buffed up and killed the worms. They appear in the same spots every time. So I moved my guys there and summoned a couple of allies.
On normal difficulty you can take a worm out with a javelin, I only have throw skill at 3, with each of me neph party. 2 to start and one from reading a book. I dunno about harder, but this made the worms cake. once the pit crawler attacked I terrorized it. That was the key for me. Keep it feared and bash it down.