Superman? A 20th-level wizard would clean his clock. Even if Superman made every saving throw roll, a high-level wizard has so many options at his command, he can easily catch Supes in a no-save spell. A single 20th-level wizard has more options, and more powerful options than the entire Justice League combined. And that's before even considering magic equipment.
Superman is vulnerable to Magic (one reason Shazam can clean his clock).
However, keep in mind that his Str, Dex, and Con are all probably in the low 60s. And his Fly speed is... well, faster than a speeding bullet.
Superman's vulnerability to magic notwithstanding, superheroes in his power category are more akin to demigods & lesser gods than even Epic level PCs. Technically, some of them are even actualy greater gods- Thor, perhaps?
As for the assertion about Supes...seeing as how he can actually attack people from beond the atmosphere (say, by dropping a 150+ ton rock on them), move faster than the eye can see (How many swift actions would he get? How many miles would his half-move be?), use his heat vision to cook them as long as he has LOS...despite his vulnerability to magic, a wizard needs to trick Kal-el into range without arousing his suspicions before he could take him down.
That said, because he
is vulnerable to magic, if he
does get surprised, it could be a short fight.
Well, that's not true. High-level D&D characters are more powerful than even comic book super heroes.
Again, that would depend upon the D&D PC and superhero involved in the discussion.
For instance, I can't think of a single D&D PC (even epic ones) who could take down
...the Flash (or Makkari). They simply don't have the reflexes to deal with any being who (in almost every incarnation) can move in reasonable percentages of
c or faster. What is a wizard going to do against someone who has landed 150 punches or more in the time it takes him to start the syllable that will cast a quickened spell...
...Or Green Lantern whose only limitations are the color yellow and the strength of his will.
...The Hulk whose strength has no upper limit...essentially infinitely stacking rage that boosts every second.
...Hyperion, Titan, Supreme, Thor, Ultra Boy, Mon-El or any of the other Superman/Shazam type characters that have appeared in comics over the ages...
...without exploiting a known weakness.
