User blog comment:IamJakuhoRaikoben/Chapter 315: Rosemary, Review./@comment-4223231-20130118233236/@comment-2214433-20130118233836

Quoting someone from a certain site:

"To everyone who is saying “where was the calamity from Kagura unsheathing her sword?”

Did no one notice pg.5? She cut past the magic barrier surrounding the Arena and effectively cut entire city in half, that was probably the strongest attack we have seen in the Grand Magical Games yet. Natsu’s ultimate technique just succeeded in blowing up the barrier, Kagura’s just cut through the barrier, the arena, and the entire city. If she got to swing that thing around, everyone would probably be dead for miles around.

Erza also didn’t block the attack, she deflected it, hence the destruction it caused."

That explanation sounds plausible imo, although I have no clue what barrier is he talking about. /wasnotpayingattentiontogmgsetup