Use a centre punch on the side of the nut / bolt / sump plug, a few good whacks with a hammer might do it, in the past I've just about destroyed the head of a bolt but it did give way in the end. Failing that a thread extractor, drill pilot hole, extractor goes in LH thread and tightens to loosen stuck item.
Thanks to Mick Charlton for the former suggestion and who I cursed at the time for being a know all but have blessed many a time since, for the things he taught me that have got me out of difficulty on jobs.