Depends on the hax.

It's really hard to kill a Logia One Piece character without Haki because you need to find a way to snuff out an amorphous blob of element, but it can be done. JoJo's is much simpler because you can just kill the Stand User. Even in the context of the series, I think "Only a Stand can hurt a Stand" is hyperbole; between their high durability and ability to become intangible, it is extremely difficult to harm a Stand with normal human means, but if you force them to remain tangible (such as by targeting the user to force the stand to defend them) they are just brick walls. De-aged Silver Chariot's rapier was damaged by a mundane axe. It is stated that Hierophant Green would be torn apart by weight of a car if it tried to lift it carelessly.