i'm going to regret this, i'm sure:
superman is not invulnerable to magic: FACT
mister mxyzptlk proved this many times. it can be well argued that superman was never harmed by mxyzptlk simply because he was a prankster who only wanted to troll superman.
however this DC incarnation of 'magic' is on par with 'Q' from ST:TNG, having near omnipotent power to alter reality at the snap of a finger. regardless, it was magic and superman was clearly physically vulnerable to mxyzptlk's will.
WITH THAT SAID, and as imp said, the nature and application potterverse "magic" is laughable to superman's ability to not even be touched by it.
superman massively butt rapes. but he IS technically vulnerable to magic. just sayin. now lets all watch quimchi act like he knew this.