Margwas have lots of health and can only be killed by destroying all three of their heads. Each head that is destroyed releases a Parasite and makes the Margwa move faster. The heads will rotate, being open periodically, with all heads being open when the Margwa first spawns, making it possible to instantly kill them with a shotgun. Maintaining distance and Pack-a-Punched shotguns are one of the best ways to kill them, with the Thundergun being the absolute best on Revelations.