She didn't defeat Proxima Midnight. Scarlet Witch did.
She didn't defeat Ultron. She defeated some of his robots.
The Chitauri are basic mooks.
It's basic Inverse Ninja Law.
The inverse ninja law is a media trope regarding not only ninjas, but any character type that is shown to attack in massed numbers, such as soldiers, robots, daleks, or vampires (but not zombies). It states that the threat level of any number of ninjas or other whatsits is inversely proportionate to their numbers. Therefore, if you're attacked by a lone ninja, you're in trouble, but if you meet an army of ninjas, they're going down.