Suggested by Destroyer Bowser Jr has a lot of little robots and gadgets at his disposal but none of them will be enough to stop Mercury. Jr’s best bet is his Shadow Mario form and even then the issue is that he has no good long range options. Mercury can destroy him from afar with her water blasts and that will be the end of the battle. Mercury’s weakness is her durability but this isn’t a weakness that Bowser Jr will be able to exploit. As a result he has no real shot here. Sailor Mercury wins.