Common problem; brake fluid is very corrosive. That's why you should regularly flush the system even if everything is working fine.
Now, to your question; I use a product called PB Blaster. It can be found at Autozone and other parts stores. It is a can of lubricant/deruster. Spray it on and it will creep into the threads. For a tough job like this it helps to lightly tap the fitting with a hammer. This breaks the rust a little allowing the penetrant to work even better. After doing this (soak it), let it sit overnight. Spray it again the next day and try to loosen it with a FLARE NUT WRENCH. If it is already rounded, you will have to resort to Vise Grips.