What tools should be used to loosen the nut on the end of a brake line?

A flare wrench is the proper tool. Looks like a box end wrench with just a slot cut through the end to go over the line. Grips on 3 sides of the nut so that hopefully it doesn't round off the sides of the nut. Some WD-40 or similar wouldn't hurt and a hammer to give the wrench a sharp tap to break contact on the nut. Vice grips if you have to.