This giant zombie acts as a boss; like all bosses, they are tougher and stronger. It's fought about 3-5 times through out all the missions together. The Giant Zombie is also encountered in the Arcade Mode in later, tougher stages. There may be one reason that they became so large is that they mutated over time in the sewers or the worms that covered them helped the process. It is recommended you start spamming your ammo out of a better gun than a sniper or crossbow which damage doesn't compensate for the time wasted.