Saturn simultaneously activates every nuclear weapon, legal and illegal, in the world and Superman disposes of every last one of them, with no loss of life. It's one of Superman's greatest feats, and a real stretch of his powers. And it's miles better than when he did practically the same thing in Superman IV: The Quest for Peace.
Superman faces Saturn and defeats him with his morally-based refusal to kill Kristin. Then, he proceeds to totally own Saturn, getting him to tell him how to save her life, to put everything back the way it was (except for erasing everyone's memory of Kristin's visit) then telling him to go to Hell, literally. This is why Superman is still the best.