i'm assuming this is drum brake and the line is alloy. I'd use reg pliers and I know it'll get buggered up, but I doubt you have a choice. If it comes off, great -- you can likely get it back together with pliers and it doesn't take a lot of torque to cinch those.
But, if not...
If you can replace the line at the coupling easy enough, just cut it and crimp the end shut so a minimal amount of fluid is lost. you're gonna need to bleed that thoroughly after you're finished, either way. you'll need a new slave unit, they are cheap enough.