be very patient with this, apply some wd 40, even some rust free. Do not manhandle this valve and do not tie on with a channel and a vice on the other side and stand on this.
Spray in some lub, then move the handle back and forth with your hand to work in the lub. Eventually it will come loose. If you apply too much muscle you will break the line, valve or whatever and have a big mess. Do not use a torch either.