Heres a tip from an old mechanic. As I have had to deal with many rounded head bolts over the years I have found this to work a treat. For obvious reasons, a single hex socket is best. Put some corse valve lapping paste on the head of the bolt before attempting to loosen it. The valve lapping paste will help it bite really hard into the bolt. It washes off. You don't want any of that stuff inside your engine though.