There are plenty of people in the galaxy that COULD take him on, but trying to get New Genesis or the Daxamites to come to our rescue would take a long time, I suppose.

I had an earlier version of this where some humans managed to escape Earth, then Luthor built a time machine and let several people use it to go back in time and warn everyone/ bring them intel/ advanced tech.

I could use that one if you think it would work better.