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wait until you fight those corrupted rock breakers
I've killed a non-corrupted one on the same quest. But he wasn't that hard as I basically stayed on a mountain so he couldn't get to me. I just dodged the thrown rocks and kept shooting. The Thunderjaw was more difficult because I was in a narrow area. There was one rock structure in the middle of the valley that I used as protection. I basically went around in circles with the thunderjaw, using the rock and shelter from his shots and charges.