DO NOT under any circumstances just hold the rifle by clamping the buttstock is a vise. This will most likely damage the bedding and/or other parts.
As NikA said, use a barrel clamp to resist the torque of removal.
I work on AR type rifles. Those muzzle devices are often torqued severely to 'clock' them against the crush washer. Most of the time, I can get them loose by using a piece of automotive heater hose to pad the barrel and clamp it in a bench vise. Sometimes, I have to get everything secured and smack the end of the wrench with a deadblow hammer to 'break' it loose. Also mentioned: A line up punch or pry bar may work but can damage the brake.