Bowser should simply take away all of the 1-UP Mushrooms from the Mushroom Kingdom and all those that are on the way to the castle, and place Kaizo troll traps, what's Mario going to do with that? Or, Alternatively, just place a fucking wall covering all of the Mushroom Kingdom, Mario won't even be able to get out, Like, Bowser can hire lots of guards, create multiple castles and boats, so why not simply seal your mortal enemy away? Or what about breaking Mario's legs? Mario won't be able to do shit without his mega powerful legs, that allow him to jump pretty high stuff that a normal human can't reach, and even if Mario's legs healed up, these won't be fully recovered, and will be unable to do his wacky incredible jumps, Also, Bowser could try to place stuff really high that Mario won't even be able to climb not even with the cat suit, or what about kidnapping Mario and Luigi? Without them being around to save the day, Bowser can do his evil plans easily, Just put them on a prison far away underground and Bowser is free to do random shit, Or how about taking over the Mushroom Kingdom by legal means and become the new king that rules over the Mushroom Kingdom? It's highly improbable due to people not really trusting him and... eh, forget this one, I had this throught line because i thought that if Bowser conquered the castle, then Mario wouldn't want to go against the law and beat him up, but i remembered that Mario doesn't give 3 shits about laws and would kill him because of his simping and shit, Also, another way to defeat Mario is to team up with that one fatass boo and make him trap Mario on some painting, then KILL E. Gadd and destroy the Poltergust, Then trap Luigi and ready, Bowser wins, Alternatively, Capture both Daisy and Peach, THEN threat Mario and Luigi that if they try to get them back, Bowser would kill the princesses, Maybe even punch the girls to make Mario & Luigi rethink their actions, and knowing how over-protective Toadsworth is, he would make Mario and Luigi turn back from even attempting, Since he'd be scared from the thought of the girls dying, Also, Bowser can put Saw-styled traps to fuck with Mario, Just put multiple original Saw-styled traps all over the castles and ready, Mario will most likely lose all of his lives and get game over'd, Also just hire some security system to watch over the castle, and then the hired mercenaries/guards would shoot Mario and Luigi to death once they see them.