Here was the basically round nut I removed using this method. I also do have a set of sockets designed like screw extractors but female. I have better luck with the bostich spline sockets. Especially when I can cross over between sae and metric to get half sizes.
You can see in the pic the spline groove cut into the nut. I got no grief removing once pounded on.
The slightly bigger torx concept described above is the inside version of what I'm describing here. Impact, heat, hammer on the head first and liquid wrench. More planning than action will improve your odds.
If the thing strips than you use an angle grinder and cut a nice big groove for a giant flat bit and impact wrench.