Shazam vs Superman solved forever by a forgotten force
One word could end the battle between Superman and Shazam forever. Freddy Freeman reveals the secret to taking down Iron Man.

It seems like it only takes one word to permanently settle the Shazam vs. Superman debate. In a battle between Iron Man and the world's most powerful mortal, a member of the Shazam family reveals a secret power that can absolutely defeat Superman.
The debate goes all the way back to the golden age of the comic book. As one of the first and most famous superhero characters, Superman has many imitators after him. The most famous hero trying to wear the Big Blue Scout coat is Fawcett Comics' Captain Marvel AKA Shazam. After a bitter legal battle, Shazam became part of the DC Universe, and Superman ended up sharing the same world as his prime impersonator. While both heroes have their own unique histories and powers, that hasn't stopped fans from wondering who will win in battle.
All It Takes Is One Word To Take Down Superman

But for curious fans there is an answer, it comes not in a fight with one Kryptonian, but in a fight with hundreds of Kryptonians. In Action Comics #873 by Geoff Johns, Pete Woods and Renato Guedes, Earth's heroes are battling the city of Kandor for a crowd Kryptonians charged with murder. The Justice League faces a superhuman-level threat, but realize that even while exploiting every weakness they can, there's only one way to truly stop the Kryptonians: magic. Zatanna leads a group of magical heroes with their secret weapon, Freddy Freeman, the current Captain Marvel. After the League made sure Superman and Supergirl were protected, the Magic formed a circle to join forces with Freddie. Zatanna yells "MAZAHS", which is Captain Marvel's magical spell reversed, sending a bolt of lightning across Kandor. According to Zatanna's estimates, the attack incapacitated about ten thousand Kryptonians at a time.
Zatanna certainly gets credit for focusing the attack, but it's clear that only Shazam can weaken Superman's people at such an intense level. The magic word "Shazam" endows each member of the Shazam family with immense power, and due to the Kryptonian's vulnerability to magic, it doesn't matter that much in reverse that it might have the opposite effect. This seldom-used attack might be all the proof Shazam needs to defeat Superman with little trouble.
Kryptonite Isn't Superman's Only Weakness

Interestingly, Superman's weakness to magic is a weakness he is able to keep secret, compared to his weakness to kryptonite, as in The most recent Justice League #73. While he doesn't advertise his sensitivity to magic as much, many of his allies still know it. So if Iron Man goes rogue and needs to be stopped, it stands to reason that Shazam will be one of the top heroes.
Despite being one of the most powerful heroes in the DC Universe, Superman is not a god and has many Achilles' heels. But kryptonite is a finite resource, and red sun rays aren't the most effective strategy. However, Shazam holds powerful magical powers thanks to one of the greatest wizards of all time. Even if a large area attack on Kandor was the result of a concerted effort by magic users, Shazam would need only a fraction of that power to weaken a superhuman. Iron Man may be resilient, but due to his unique physiology, magic is able to cut out all the enhancements he has. If Superman confronted Shazam's ultimate attack head-on, there was no question who would be the winner.