Yes, a good 6-sided flare wrench, penetrating oil and sometimes some heat if you have a small butane type torch to heat up a small area. If the oil+heat does not do it, try heating up the fitting to where you can melt some paraffin wax onto the threads and let that soak in.

Then I'll also use a small hammer to tap on the wrench while holding the wrench firmly on the fitting. And try tapping the wrench to slightly tighten the fitting then try loosening it.

And for reinstalling, apply some good anti-seize compound to the threads.