You did a good no on that one for sure. I think I would go with a socket type of tool with a sturdy hex bit in the end of it. You would get more torque this way in unloosening this problem screw. Maybe just cut the long end off and put it in a 1/4 socket. I create tools all the time by making up what I don't have. Allen keys are cheap to buy and easy to alter as well to fit most smaller socket sets.