Thanks, that did the trick. I still wonder why my tearblast arrows didn't work, though. It specifically says "tear" as a weakness for the digging arm and I even modified my bow to have like 200 tear. But literally nothing happened when I hit the arm.
I think I hit the same arm multiple times and it didn't seem to have any effect. But maybe I just missed. Or maybe it was a bug.
I did this exact mission last night and noticed the same thing. I hit the same claw 3 times with my near max-Tear Tearblast arrows and it never came off. Instead a bunch of random armor plates were coming off.
I had an overridden Shell-Walker and Ravager on my side providing help and distractions but otherwise just kept hitting them with Triple Fire arrows and that melted them. They didn't come near me either as I was far enough away I suppose.
But my MVP Shell-Walker walked right up to them up the hill and survived the whole ordeal with 5% health, haha.