Since Dela-Ray is human she has powers to help her survive in a cold planet. If the Mercurians came to earth they would have some sort of power to keep them cold. Dela-Ray's is that she can make fire from her hands. She does not need a source. Fire melts the Moon people. The Moon is a cold place. Since Mercury is also cold the Moon people do not get powers they have to hit you to apply damage. If you melt and destroy 10 Moon people they will surrender and you will win, but if they kill you you lose and they will take Mercury over for trying to help humans and then who ever tried to help Mercury will die until it is just the Moon people.