Cut the hard line and replace both. The tubing nut is already mangled anyway.

Get the correct size tube assembly and start over... Note usually there is not enough length of the tube to reflair...

If you can, take the other end of the brake line off and slip a box end wrench over the brake line. The box end will give you more grip on the frozen fitting

I was in this pickle with my 84 GTI when upgrading to braided stainless hoses. And to make it more frustrating, it was under the floor pan near the beam axle pivot in the rear. My solution was to obtain a inexpensive 12 point box end wrench and with a Dremel cutoff wheel, cut a space in the box end just enough to slide over the hard tubing. Then with some penetrating oil and heat, it snapped loose while saving the flare nut. The wrench was a cheap solution and I keep it for just that metric usage. BTW, I have regular tubing wrenches, but many are too thick to fit in tight places. 
 Pintosopher, A stubborn horse is yet the Mule!
 An addendum, If the wrench tries to spread and round off the corners of the tubing nut, Consider installing a Stainless fuel hose screw clamp just to hold the ends snug on the wrench over the tubing nut while breaking it loose. Then remove the clamp and proceed..