Whats the best way to get this fitting off

I'd do as some have suggested, grind a few grooves inside until they just start touching threads, but I'd use a die grinder with a rotary file. This will relieve some of the compression tension and will aid in removing it. Most of the time if they have pipe dope or teflon tape on them they aren't too bad. I'd be careful with how much heat I put on aluminum.