During his early appearances, Killer Croc was just a colourful nickname that stuck to a burly criminal with a skin disorder which made him look a bit scaly and scary. In post-Crisis On Infinite Earths continuity, meanwhile, hes literally a giant mutant man with sharp teeth, impenetrable scaled skin and the like. Hes a half-man, half-crocodile. Meanwhile, despite his name, Batman is not half-bat, half-man. Hes just a dude, of regular size and slightly above average stature, with a belt full of Bat-Shark Repellents and the like. And yet Killer Croc is never considered anything more than a C-list villain at best, often escaping to the sewers to be quickly dispatched. There is no way Batman should be able to win any fight against Killer Croc. He is a giant crocodile man. As soon as he gets his hands on Bruce Waynes frail, delicate, fleshy body, he should just bite his head off. No matter how fast Batman is, he cant hurt the guy with scale armour all over his body. He just shouldnt be able to win.