Serious help needed
If you can work some flat spots on what's left of the nut with a file until you can barely get a 6 point socket of what ever size that you can hammer onto what's left and then use a breaker bar attached to the socket and smack the end of it with a hammer, we call that procedure (shocking it) the vibration will work in your favor.
This works about 70% of the time. Good luck.
This works about 70% of the time. Good luck.

