He didn't cut through the sledgehammer because Hale wasn't constantly attacking. He had the advantage that Kami was busy with Mithias, and he specifically stated that Hale only attacked when he saw the perfect chance to do so. Had he attacked continuously, Kami would have chopped that thing in half with ease.

The only reason Mithias could prevent his blades from being cut is because of his superior speed and strength, as well as his experience as a trained swordsman. He used to be a templar, and as such was trained from birth in the use of a sword.