Cracks Thanos's sword, shatters a part of it at the other end with a punch that also sends him back, despite all his strength.
Seizes Thanos, crumples his armor, overpowers his arm with a swipe of her finger, nearly kills him
"Wanda Maximoff, who is probably near the upper echelons of power -- I contend she would have taken down Thanos if he hadn’t called the [army], it was done"
“If you look at Endgame, Wanda Maximoff was going to kill Thanos. If he hadn’t desperately — that’s as scared as I’ve ever seen Thanos. And if he hadn’t said, ‘Decimate my entire team to get her off of me,’ I think she would’ve done it.” - Feige
Seen as powerful enough to bust his helmet with a literal tap.
Written to one-shot him with a single strike
"So that was quite a process for us, to develop the finer details of what this effect would look like. And always keeping in mind that, while it was definitely way more amped up than we'd previously seen it, it still had to be recognizably the same kind of magic power that we'd seen from Scarlet Witch in previous films."
...But the real challenge was in creating Scarlet Witch’s magic power. Scarlet Witch takes her powers to the next level in ENDGAME. For Wanda, it is only 5 minutes since Thanos killed the love of her life, Vision, and so she is seething with anger. Since she first appeared in the MCU, Scarlet Witch has been developing her powers and in ENDGAME she becomes powerful enough to triumph over Thanos, stripping the armor right off him"
"She's understanding it more, and she's able to use it to an even greater effect. And that sort of really comes to a head in the end battle in Endgame where she's really kind of deadly to Thanos. She's able to really take control." - Matt Aitken, VFX supervisor