Remove over tighten copper pipe fitting
First, check the basics:


Are you turning the right direction?  It is easy to get backwards when you are in an unusual approach.  Inspect the thread closely for the correct loosen direction.
How long is the wrench?  An 8 inch wrench is probably insufficient for loosening 1/2 inch copper fittings, though it probably is sufficient for tightening them.  Try a 12 or 16 inch wrench.  The greater leverage of a longer wrench provides greater torque, but more importantly provides more control at that torque so you won't be as tempted to "pulse" the torque.


To protect the valve and safeguard other plumbing, be sure to provide counter-force at the other side of the joint with a similar wrench or vicegrips, etc.

In my experience, WD-40 is no help with plumbing.