Thanks for purchasing/playing! The zombies' attacks were designed to distinguish them from the spiders, who are also melee. Use grenades to soften them up for the killing blow, and also try to accumulate armor; armor applies to each of their claw swipes, so the health you can save increases dramatically with just one or two armor upgrades. Another tactic would be to build a medium-damage shotgun, which can be used to sweep the halls and kill them in usually one hit.