Metal brake line nut won't budge....
 

Lots of localized heat from a propane torch like "Bernz-o-matic" should do the trick. Let it cool down slightly then try. Might take a few heating and cooling cycles.

Use a longer lever. Slip a length of pipe over each wrench to give you more leverage. Those parts look very clean, they're not rusted together, so penetrating oil won't help. As long as you're counter-holding the other side and you're using proper flare nut wrenches (as you are), it will break free.

Remove the lot from the bracket on the body along with the flexi hose as well . As above spray it first and then give it a little heat to cook it inside .

Sometimes, for no apparent reason, things just won't come apart. Are any of the fittings stainless steel? Failing the "longer lever" approach you might have to sacrifice the flexible line by cutting the rubber tubing close enough to the fitting so as to allow a socket to be placed over it.

Put a vice grip over the flare nut wrench head while it's installed on the nut to keep it tight unless you are extremely confident in the quality of said wrench. They can and will spring open under enough pressure and round the fastener. Once that's addressed, as was said little heat and make sure the other side is held equally firm and use more leverage. It'll break, just don't let it slip.

I have found that my knipex cobra head pliers will make quick work of that.

if you have a line wrench then use a longer lever or find a crows foot line wrench and use a breaker bar on that - dont ease on the wrench, go for sudden smacks on it.

I'd be very nervous about using heat unless you are tossing the flexline, however yours looks new.
The tips about a sudden smack and the vice grips to keep the wrench from spreading are good points.

I second this.

Heat has always been my preferred method, but getting it hot enough makes the fluid boil and that pressure needs somewhere to go, so I've always cut the flex hose before applying the heat; perhaps you can dismount the caliper and spin it off the end of the hose first.