Good flare wrenches and a lot of heat! If it is the caliper end, heat the junction of the caliper more than the nut. Make sure that the ABS sensor wires are out of the flame path.

Red hot is good, then put the flare wrench on it and tap it lightly. It should come right off.
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Keeping it running is better than buying new

Removing brake fittings can be a PIA.
Soak with Mystery oil (don't confuse this with WD40) and let it soak first.
get the oil on the brake line and the fitting threads.
This is best done a day or so earlier, the penetrating oil needs time to
work.

You need high quality flare wrenches
Some people swear by Snap-on, but I've got by OK with Craftsman.

Once you get the wrench on the fitting, you might try tapping the wrench
with a small ball peen hammer. Shock helps release the frozen fittings.
You need to avoid so much force that you deform the fitting, at which time
things get more ugly.

What exact fittings are you working on?

Bill