Couple things you can try
1 Soak in penetrating oil overnight
2 try tightening until the fitting starts to move (will break the fitting away from the line) then work back and forth to loosen
3 little bit of heat with an oxy /acet. Torch and work back and forth to loosen
Cutting and flaring if all else fails
Hope it is some help
One trick is to get a ring wrench onto it, by undoing the other end and working the wrench along the pipe. Had to this on the TR8 recently. Takes some patience if the other end is at the other end of the car!
PB Blaster works much better than WD40. Spray regularly over a day or two.
I had similar issues when I was replacing my brake lines with ss lines. I ended up having the replace two steel lines from the ABS modulator to the jump line that go to the rear steel lines and the front ds line. I also has to cut and reflare one of the rear steel lines where the jump lines connects to.
PB Blaster and heat didn't help. I ended up shearing the threads off the fitting on the steel line.
If you decide to cut and flare. You need a bubble flare tool.