In the fifth mission, you encounter red barrels. You can aim your crosshairs over one to automatically shoot it (like you do with zombies) and it’ll explode. To get the most out of these, let the zombies that are coming from the red barrel’s vicinity mass up, and then shoot the zombies that are not near it. When the side with more zombies is larger, shoot the barrel to explode the mass. The explosion radius is huge. You could also “train” the zombies in circles until you have a large horde of them, then lead them toward the barrel and shoot it. Red tanks and fire extinguishers can also be blown up, but the fire extinguishers have a smaller explosion radius.
