If you have a pressure gauge with a bleeder for clearing air from the system, you may want to see if there is any pressure built up by bleeding it. There may be enough latent pressure to make it hard to break free. Just a thought.
Mine was hard to get off too. I used a semi-heavy rubber mallet hammer, put on the octagonal wrench, and carefully hit the handle of the octagonal wrench. It took a little while but it finally busted loose.