LOL @ a lot of these responses.
You'd have to get a fighter into a time capsule til an era where that fighter was 4-5 years old (Floyd started fighting around then) and train him in a similar or BETTER manner than Floyd was trained. Teaching that fighter everything in boxing skills and boxing knowledge. When that fighter turns 18 or 21, bring that fighter back into the time capsule to this very day.
Now you finally have a fighter that can defeat Floyd Mayweather or at the very least give him the greatest nightmare of his life. Unfortunately, there is no fighter today that can match him in boxing skills and ring IQ. The next best technical boxer with a high ring IQ today, is Marquez. And then Andre Ward. But it remains to be seen if Ward has a boxing IQ that is even as good as Marquez.