You should be able to unscrew the fixture from it's bracket, just twist the whole thing counterclockwise. Do you have any intention of saving this fixture? Personally, I'd just smack it with the big hammer until it's down.
If you want to save it, remove that tiny allen screw and see if that brass piece moves. My guess would be that the brass piece slides back, then the cover piece can be pulled back, then the two nuts can be loosened so they're not jammed together, and then the threaded nipple oughta unscrew.
But like I said, if that sounds like too much screwing around, I haven't yet encountered a light fixture that can't be removed with a large hammer...