Board Thread:General Discussion/@comment-32.208.226.199-20160526161241/@comment-10380701-20160629000638

It's late for the OP, but aura has limitations. It increases one's overall attack and defense but it's akin to an energy shield. As the user takes more damage it decreases their aura until they run out.

In the tournament, we never really reach the zero aura point, but presumably, Pyrrha ran out during her battle with Cinder, making her easy pickings by then.