FWIW, I deal with this a lot at work and for the really tough seals we have to use a combination of T-tape AND pipe dope. Like for SS screw-plug type heating elements into SS tank fittings...in a hot caustic tank...a real ***** to seal sometimes.
The regulator port should (key word; 'should') have flats on it such that you can put a wrench on it...then another wrench on the flats of the valve. That way you can hold the regulator securely while you thread the valve off. It usually works better/easier if you can align the wrenches such that the handles are close together...then you just grab both handles and squeeze them together to get it going. Once you 'break' that strong connection with the pipe dope/Loctite; it should thread off fairly easily after that.
Using a full bottle of CO2 as something to hold so you can get the valve off is questionable regarding safety.