If the lug bolt has any sides left that can still grip the bolt if a socket is place on it...you can give it one last try by using leverage. By that I mean a long torque bar used with the socket...the longer the torque bar the better. The longer the bar...the easier it will be to loosen the lug bolt. If you have any metal piping...you could use that by slipping it over the handle of your socket wrench handle.
I carry a metal pipe (in my trunk/cargo area) that is about 3½ ft long due to past experiences with over-tightened lug bolts. I guess it can come in handy if I need to crack a few skulls, too.
I had this same issue as the OP when I first purchased my f06 650iX GC. And it was discovered when trying to mount my winter tires for the first time. The front right wheel lug bolts were over-tightened. We were able to get 4 of them off manually by using a 6ft steel pipe over the handle of the wheel wrench. The 6 ft pole moved the lug bolts like butter...after failed attempt using a smaller hand wrench and quickly determining that the impact wrench wasn't doing anything but destroying the lug bolt. The 5th one still wouldn't yield because it had got rounded down using the impact wrench.
The end result after multiple attempts to get the remaining bolt off (heat & welding)...the wheel had to eventually be destroyed...along with the wheel hub. I ended up needing a new wheel hub and a new Style 356 wheel. The work was done by a local Indy shop that tried to do everything possible to do as little damage as they could.
The only good that came from this experience is that I ended up power coating my Style 356 wheels so that they all would match look the same. You can see the damaged area of the original wheel (highlighted in red) in the pic below...compared to the new powder coating.