The Rex's armor was perfect for deflecting high-velocity projectiles and protecting the user against explosive rounds. There was no way conventional kinetic weapons were going to put a dent in that thing.
Kevin realized that ranged attacks weren't going to win him the duel, so he switched his strategy and prepared to close the distance. With his tower shield shattered to pieces, he had little left in the way of defense, which is why he decided to go on the offensive this time.
Having exhausted all his grenades, Gunther too was forced to switch to his secondary weapons. It allowed Kevin to charge straight at the Rex, without any hesitation. As soon as he was in melee range, he went for the Rex's helmet. Raising his war hammer above his head, he took a wide swing, bringing the weapon down with such force that Gunther felt the blow even inside the exosuit.
