Well it depends on what lengths you've gone to and want to go to...

Im guessing you've tried mole grips etc and had no luck?

Has it got any of the Nut body shape left or have all corners rounded off?
If some are left, get an impact Hex socket (the socket is the same shape and the plug head) this might help get it off.

Worse case scenario, tack a small length of steel rod/bar to the sump plug such that it creates a lever, then undo the plug........

Good luck