What sometimes works for me is "tapping the wrench". Instead of rounding the nut by simply bearing down, soak it for several days in penetrating oil. Then sort of tap the wrench...a little quick pressure, release, pressure, release, etc. The wrench should "click" each time. The same way an air wrench works to loosen a nut. In the worst case, you may need a new hard line.

Vise grips make decent welding clamps. Other than that, I've never found a use for them.