Marvel Comics’ Doctor Strange and Scarlet Witch are two of the most powerful magic users in the Marvel Universe, attributes that would mean their downfall if they ever went up against the hero from Shonen Jump's Black Clover's hero. While Stephen Strange and Wanda Maximoff have near unlimited power, it would be for not if they fought a young boy whose power revolves around the antithesis of magic.
Asta is the protagonist of the manga series Black Clover by Yuki Tabata who, in a magical world, is not born with magical abilities. However, at a young age, Asta receives a mysterious five-leaf clover grimoire with an anti-magic devil hidden inside. The devil possesses Asta and grants him powers and techniques unlike any of his peers. One of the abilities he is granted is anti-magic, a unique energy blast that nullifies all surrounding magic and magic users.
Doctor Strange is the Sorcerer Supreme in the Marvel Universe, besting other magical entities across the multiverse including Nightmare and Dormammu, proving his superiority in the way of the mystic arts. Scarlet Witch is an incredibly powerful hero gifted with reality manipulation as well as a powerful sorcerer, having been responsible for obliterating countless mutant lives across the globe in her most infamous storyline within House of M. While the two Marvel champions are insanely powerful, especially when working together, the previously stated power of Asta would nerf them in a moment.
While Asta would most definitely win by taking away Doctor Strange and Scarlet Witch’s powers, they can’t be completely counted out of the fight purely based on that fact alone. Doctor Strange is a literal genius who not only has mastered the mystic arts, but has an intellect hitherto undreamt of. Wanda has been a long time member of the Avengers as well as having worked under her father, Magneto, on more than one occasion and has assuredly picked up on battle tactics. While the two Marvel heroes would give it their best efforts to the end after their power would be taken away, unfortunately for them they would still lose as anti-magic is just one of the powers in Asta’s arsenal.
Asta has enhanced speed, strength, endurance, and is a master swordsman. The second he would use anti-magic against Doctor Strange and Scarlet Witch, he would quickly and ferociously cut them down before the two could even blink. To make his victory even more assured, Asta also has the ability to bargain with the devil that possessed him, trading, for instance, his arm for more power with that arm being molded into a demonic form, offering Asta even more abilities. Doctor Strange and Scarlet Witch would lose against the hero of Shonen Jump's Black Clover, Asta, and it wouldn’t even be close.