A bit of a wire brush off and a squirt
A tap tap here
And a tap tap there
A big flat ended punch, in this case a bolt was used. And a hammer.
Best if you can hit two apposing flats on the hex of the flare nut. Each application will differ. Pick the easiest ones to get to, obviously, and if you can't get to opposing flats, any 2 you can get to will do, you just might have to tap a bit harder.
A shifter AAAAAAHHHHGGGGGGGG!!!!!!!!!!!!
RUFF AS GUTS!!!!........read on 
A good shifter is better than an open ender for a couple of reasons:
The jaws are wider.
Once fitted to the flare nut, you can tighten the shifter jaws to get the best fit on the flare nut.
A worn out shifter will be as useless as an open ender.
How the jaws close up together and how much the adjustable jaw wobbles is an indication of it's suitability.
Believe it?......
Or not!..........
