I have a like new 15 lbs bottle that needs the leaky bottle valve changed to my new one...I don't want to scratch the heck out of it, so does anyone have a trick for changing the valve out?

Problem is...I can't hold the bottle tight enough to get the old valve to break loose. I tried holding it while someone else used the wrench on the valve...no go. And the vottle wants to spin in it's mount when I tried it that way.

Also...do I need to put liquid teflon on the threads when I install the new one?