User blog comment:IamJakuhoRaikoben/Chapter 379: Tartarus Arc, Part 3: Underworld King, Review./@comment-17764746-20140404080519/@comment-24066070-20140404150423

Since spears fail to injure these demons, it makes sense to try something else. Natsu repeatedly smacked Fran with that boulder and he seems just unconscious, it's normal to be unconscious when you're getting whacked on the head (I take it Fran's entire body counts as head) with something.

Still, I do think Natsu deserves the Boulder Badge for his performance.