This happened when our ally of 16 or so was down to myself (weakened) and a WHM, which is why it was so awesome that he died. On top of that, he had used physical shield a little bit before CB, and luckily it wore just in time.
@Superman: Normally, one would pop a chest after defeating Proto-Ultima... Therefore, the drops would be in there. He might be a *** for some people to fight, but he's curtious enough to gift wrap his presents.