So I found it easiest to use a piece of wood, or a cutting board like I did for this jig. An important part is to have a table vice that's large enough to hold your upper receiver.
Cut a notch into the board to fit the front tang of the receiver. I've made mine to fit both AEGs and real steel for WE and PTWs.
This is what it looks like in my Modify table vice. The import thing is to support the front end of the receiver. This is the thickest and strongest area to support.
You can also use a standard table vice but you'll need to make two boards. One for the upper and lower so you don't scratch your finish. Always make sure that the front thickest part is clamped tight. Then you can use more torque to remove stubborn barrel nuts. Don't do what I did here as you can see the very front is not supported. This is just to show.