What Kurt said.Start soaking the castle nut now with some kind of oil, screw-loose, etc.
My first couple tries to loosen the castle nut, it would not budge. I soaked it every time I walked by it for 4-5 days. It came off pretty easy after the penetrating oil soak. Put the gas cylinder wrench on the gas cylinder. It is splined to the barrel and will give you the ability to apply torque to the castle nut without trying to get your wife to hold the rifle down.