Ok! So AMAZO! A lot of people don't know about him so I'll use the official description of him here:
If you don't want to read all that, basically, Amazo is a robot that can learn anyone's ability. He could easily defeat Superman due to having all Superman's strengths, but eliminating his weaknesses.
Basically Amazo is super OP, anyone he fights he can learn how they do what they do, and replicate it... so One Punch Man?
Amazo would simply learn what One Punch Man can do, and then use all the other abilities he's gained to defeat him on top of using One Punch Man's existing abilities.
Pretty insane yeah?