I'd get a small container like a medicine bottle and fill it with some kind of light oil like ATF or if you really want to have fun, PB Blaster. Let it sit overnight then take the brake hose and put that big nut into a vice and have fun with a pair of vice grips on the broken fitting.
You can probably get the end of the brake line flared again and have a new fitting on there or you might take the opportunity to replace the whole thing.