The trick I believe is to get the copper up to the hard solder melt
temp before the oxidation catches up. It's a little more difficult
that working with sterling.
It is not more difficult to work with copper than with silver. Copper
simply is a different metal and requires different technique.
The solder for copper must in rod form of the same gage as metal been
soldered. 10% silver, 5 to 10% zinc, and the rest copper, makes
decent solder. If commercial flux is not available, regular borax
will do. Fancy additives are there to compensate for errors in
Borax should be in powered form, no liquids, no paste. Warm up solder
rod and dip it into borax. Solder must be hot enough to melt borax
and have it adhere to it.
Bring joint to red heat using reducing flame. Flame must be such that
held at certain distance metal must appear clean and shiny. If torch
moved further the area would blacken, moving it closer again would
restore the shine. Copper must be heated in that zone. Observe the
metal while heating. First it will oxide, but than oxidation will be
gone and metal appear clean. Solder must be applied at this moment
simply by touching the area with point of solder rod covered in
borax. It should flow nice and easy and leave bright joint behind.