For that (hopefully rare) occasion when you truely butcher a hex nut fitting and round off all the corners beyond any hope of getting a grip with any normal wrench, even a flare nut wrench, here is a solution that could save your britches. You will have to sacrifice a good wrench (make it a cheap one), but it may be the only way out of a jam. Starting with a 12-point thin wall box end wrench, cut a narrow notch out of the end to break the ring. This will destroy the normal function of a box end wrench, because it will lose a lot of rigidity with the ring cut open. Slip this seemingly worthless tool over the damaged hex fitting, then clamp a small locking plier onto the ring of the tool to constrict the metal onto the damaged fitting for a good grip. This way you can get a really tight grip on a hex fitting without smashing it, even when there are no visible corners left on the fitting.