If the joint is corroded or was overtightened in the past you may end up damaging the adjacent section of "hard line" in the process of removing the rubber section. Some times this is unavoidable. Spray the joint with "Kroil" or some other penetrating oil (don't use WD-40) and let it soak a day or so. Use quality tube wrenches on the fittings. May sure you're turning the wrench the correct direction for loosening. One other trick is to first attempt to slightly tighten the fitting and then loosen it. If the fitting is badly seized, the fitting nut corners may round before the fitting breaks free. The next step is to then use a set of small visegrips on the rounded fittings. If that doesn't work then one may have to cut the joint apart. In any event, the hardline section will have to be replaced. Pelican can get you these hard line pieces.
Jim