The Situation: Based on the H.G. Wells novel, War of the Worlds (1953) features a Martian invasion of the United States. Firing weapons at their ships doesn't work. Fighting them doesn't work. Our world is close to destruction until...
The Solution: The common cold. Some of the best defenses are the unlikely ones and a common virus takes down the invading Martians who aren't accustomed to the viruses that are prominent on this planet.
World-saving advice: The next time aliens attack, don't forget to sneeze.