Strange Adventures #82 (1957)

On a distant planet, an alien named Xoll is tasked with the job of traveling to Earth to evaluate its defenses. This task must be completed to better prepare for an invasion of the planet. Xoll is instructed to subject himself to Earth weapons but return back alive to report his findings. Xoll travels to Earth and begins to cause mayhem. The humans respond with conventional weapons but none have an effect until they use atomic warheads. Xoll is hurt but manages to return back to his home planet to file his report. In the meantime, the humans gather to discuss what has happened. A scientist named Albert Prentiss correctly surmises that the alien was sent to test their weapons. He also theorizes that his cosmic cannon might have an effect on the alien who will most assuredly return for an invasion. The anticipated day arrives and the aliens invade. They build an immunity to atomic weapons and assume that Earth has no other defenses, Prentiss confronts the aliens with his weapon but it has no effect at a distance. The weapon has to be applied to the aliens point blank. Prentiss is successful in discharging his cannon up close and the aliens flee for their lives