Flare nut wrenches just help you avoid rounding off the nuts -- they contact more flats, spreading out the force. Harbor Freight sells a perfectly usable set, if you have one of their outlets near by.
If there's room, it's sometimes helpful to tap the end of the wrench with a rubber mallet, in the loosening direction. Sometimes the impact will break loose a stuck nut without rounding it off, when straight-up torque won't. Lots of smaller taps are better than one big one. I use this technique all the time, especially when dissimilar metals are threaded together (e.g., a steel bolt threaded into aluminum tends to stick.)
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.