Getting around to replacing the rear struts but gott stopped dead in my tracks when I found the brake line fitting mounted in the rear wheel well (coupling the flex line to the rigid line) frozen.

Torquing the the upper fitting, I noticed the metal line turning with the fitting. I used liquid wrench and stopped when I rounded off the first corner on the fitting.

I was thinking about removing the retainer clip and urging the fitting down slightly so I could turn the 18mm hex while just holding the 10mm fitting steady until it breaks free.

Any suggestions?

You're on the right path. Remove the retainer clip and hold both ends. Those flare nuts (the ones on the rigid line side) are pretty notorious for rounding out. You might have to use vice grips on them.

If you cant get the nut to turn free of the line you might be able to simply replace that section of hardline (I assume we're talking about a 240z). There is a junction bloc in the back of the car that you can hopefully get the line to come out of without too much effort then just buy a section of hardline at napa or wherever with the proper fittings.