Two important factors about his strength & endurance.
(1) Superman's greatest feats of strength have happened while he's been flying. This leads to the idea that he's using a different power source while performing such a feat. You'd think someone often described as a 'planet mover' would be virtually unmatched - but non-flying bricks like Darkseid (OK he can levitate), Mongul, Doomsday, Orion and Solomon Grundy have all matched - if not exceeded - his strength levels h2h on some occasions.
(2) His powers generate a microscopic layer over his skin that is used for a few purposes. He has great enough durability to tactically use his body as a shield - to help others and try to convince somebody to surrender - but a magic-blessed weapon can cut right through his skin-level aura like there's nothing there. How deep the cut depends on the writer. Wonder Woman's tiara and sword have maimed him; she probably could decapitate him if she was bloodlusted. Only other things that can do the same are connected to green kryptonite & red solar radiation.