She didn't summon a giant Skeleton, she summoned a Gashadokuro while similar in appearance to a giant skeleton it's much deadlier. Firstly it's a giant skeleton made of smaller skeletons of people who died of starvation, secondly it's entirely silent despite it's size. The only sign it's there beyond seeing it is a strange ringing in your ears, at midnight they appear to find travelers and bite their heads off.

Ancient Japanse Stands, i always knew Araki was immortal.