Avengers' Scarlet Witch has already proved she can defeat the Hulk, but only under a specific circumstance. Bruce Banner’s infamous green alter-ego constantly proves that he is the strongest one there is, having survived military weapons, superhuman conflicts, and even powerful dark magic from Earth’s most powerful sorcerers. Nevertheless, Wanda has already figured out how to use her magic to end threats from gamma-powered foes.
In Avengers #72, Jennifer Walters, aka the She-Hulk, is on the run from the Avengers following an incident where Jack of Hearts absorbed her gamma radiation, thus causing her to revert to a savage and uncontrollable She-Hulk similar in power and temperament to her elusive cousin, Bruce Banner. When the Avengers track her down to a small town in Idaho, the terrified and lonely Jennifer finds the town evacuated with only Wanda Maximoff present, offering to help her former teammate gain control of her current situation. While Scarlet Witch promises to not upset or anger her, Jen reveals that it is extreme fear which triggers her transformation, becoming She-Hulk and then attacking Wanda and the other Avengers nearby.
In Avengers #73 by Geoff Johns and Scott Kolins, Scarlet Witch dodges She-Hulk's attack amidst the debris and rubble caused by the ensuing destruction. Seeking to appeal to Jennifer instead of She-Hulk, Wanda uses her chaos magic to age the gamma radioactivity in Jen’s body until it is inert, thus returning her to human form. The process is initially successful, but when Jennifer realizes she is becoming weaker, a fresh burst of fear causes her to Hulk out again, bringing the building down on them both.
The gamma radioactivity in Bruce Banner’s bloodstream that allows him to transform into the Hulk has made him one of Marvel’s powerhouses, which can be troublesome when he becomes too much for other heroes to handle. While conventional weapons have little to no effect on him, magic generally falls into that category as well. When Doctor Strange sent him to other dimensions, hit him with powerful arcane spells or even dabbled in dark magic to fight him physically in World War Hulk, the Hulk either shrugged it off or was temporarily incapacitated before returning for revenge. While magic seems like it would be the silver bullet to stop Hulk, gamma energy has mystic qualities of its own, and this - combined with his sheer power - often sees him triumph in such encounters.
In contrast, Scarlet Witch tries to use her powers in a clever way, aging the gamma that gives She-Hulk the same powers so she’ll become human again. She-Hulk gained her powers from Bruce's blood and, in this moment, is noted to be close to his level of power, suggesting Scarlet Witch's spell could stop both Bruce and Jennifer - but only if she can avoid aggravating them in the meantime. A stealthy application of Scarlet Witch's magic could take down a Hulk, but if they realize what she's up to, their emotional reaction is likely to be enough to give them a chance to stop her. However, considering the Avengers were barely able to stop She-Hulk even with the unexpected intervention of Bruce himself, Scarlet Witch may be a little hesitant to try again considering the consequences if she only angers the Hulk more.