These giant brutes are the ones with massive jaws on their chests and a sword arm. They have the worst hearing and sight range and are incredibly easy to outrun, however, they can take a hell of a beating and will easily kill you with 1-2 hits for the unlucky. Best fought at range.
Piercers are easy to kill with a rapid-fire gun. Just walk backward and fire.
Another method for killing piercers is to use 3-5 Molotov and bear traps.