When a bolt or nut comes to me burred like that, I file the flats carefully until it is a tight fit on the next sized socket, which would be 15 mm in this case. Use a six point socket on an impact wrench. Don't waste your time with 12 point sockets for any bolt that is done up tight, you just spoil the hexagon. And don't waste your time with a breaker bar, either. You'll be surprised at the remarkable difference the right gear makes. Once you try it, you'll never spoil a hexagon like that again.
