Story questions
I am not so sure about that first part and am actually curious myself since I missed the first part of this event due to work.
As far as hurting the Karka, I think it had something to do with the guns we could pick up from the weapon stacks. They seemed to have some sort of acidic property that would break through their shells. I don’t know if I am right with that but that is my guess. Sorry I couldn’t be more help.