Supes isn't weak to magic, he just has normal defenses to it. He fares as well as any normal human against it... well, any normal human as resilient and tough as he is. Getting a fireball to the face isn't going to do much because burns that would kill a human will only inconvenience a Kryptonian.
Also keep in mind all magic users he faces are insanely high level. Don't expect Rincewind to do much. :snigger:
Also keep in mind all magic users he faces are insanely high level. Don't expect Rincewind to do much. :snigger: