The weapon art on the Ultra Greatsword gives you hyper armor. In general some attacks on the Greatswords have armor in that they can't be interrupted by attacks. That's why in a hits trading battle you will always win out, you can time your attacks to their's, tank the attack and hit with an attack stringer than theirs.I have no idea what hyper armor is. I don't remember it being a thing when I played DS3.
Also get a Hornet Ring and see the massive damage you get from viscerals.