take the engine out and swap the sump?


but seriously, if you can get a smaller socket on the end, make sure it's only got 6 sides to it (as someone posted the pictuers of up top), put a fairly long extension on the end of it (to make sure you are keeping the socket properly onto the nut) and then get a big long bar (so you can apply even pressure without struggling and possibly slipping off the end if the nut decides to give up the ghost)

the other option, if you want to try it, is to use an impact drive to knock it off

you can get all sorts of tools to undo rounded nuts but they don't tend to work as well as they do on the telly

if you can find a stud extractor that might be worth trying too

one more thing
if you are worried about drilling through the bolt to get an easy out in there, don't worry, if you buy a new sump plug you can measure how deep you can get away with drilling before you go into the sump itself (not that you'd want to be doing that and getting swarf into the sump now do you, then you WOULD have to change the sump just to be on the safe side)

and one final thought, try using an impact socket on the nut as they have thicker sidewalls and have less chance of cracking