How I used to do it....

Discover the sump plug is rounded,
Use a good 6 sided shallow socket and make sure it's on the plug fully.
If that failed I'd try a 12 sided 1/2 inch socket followed by a 6 sided 1/2 inch socket hammered onto the plug.
If that failed, I'd use a good set of vise grips really tight.
Failing that it was the hammer and chisel.