Get a Vise Grips 4LW Locking Wrench. This particular Vise Grips tool will hold the hex-head fitting on the hard line securely WITHOUT damaging it. The LW series is unlike any other Vise Grips product. I have all three, but the only one I ever use is the little one.

Using the 4LW, clamp it down on the hard line fitting so you can hold it fast. Cut the hose at the connection to the hard line so you can get a deep 6-point 10mm socket on the hose fitting.

Hold the locked-on 4LW steady while turning counterclockwise on the socket wrench (I like to use a long-handled 3/8-drive ratchet). You do NOT want to apply rotational torque to the hard line fitting. Hold it fast or you run the risk of kinking the tubing.

I have removed 40-year-old brake hoses using this method. Haven't lost a hard line yet.