Unless you remove the supply line, you won't be successful with removing that angle stop. You then place a wrench on the copper colored hex, and another wrench on the chrome colored hex. Keep copper colored wrench stable, and turn the chrome colored hex in a counter clockwise rotation.
And again, make sure water is shut off and open a faucet to relieve any pressure before you remove anything.
When I was a young apprentice an old timer taught me a simple trick when using two wrenches on something like that.
He told me one hand is stronger than two. I had no idea what he was talking about and I had a wrench on each side and was trying to get something loose. He showed me to put both wrenches on the same side and at a slight angle such that I could put my hand around both handles at the same time and squeeze. Angle them on way to tighten and the other to loosen.
Doing this with one hand squeezing makes a lot of force and if you need even more squeeze with both hands.
The advantage with something like you have is the force is not going to be stressing the thin copper stub coming out of the wall all the force is contained in the two fittings.