Brutality

Firebending, even MC's style of it, needs to touch the ground a lot more than airbending. She'd get enough glimpses of his moves to know what he's probably gonna do and prepare accordingly. Aang doesn't touch the ground nearly as much and as such she gets far fewer pictures of what he's doing.

She is canonically just built like that. Nothing more to it.

How so? By that logic, if you have dodge a child running at you with a knife than the child isn't weak compared to you. Finishing a fight in a second is literally his whole thing. That's like saying someone is as strong as the Flash because the Flash needed to use his super speed to dodge a bullet from their gun. It doesn't take real effort from Kashimo to inturrupt her casting of the domain or get out of it's range, she's too weak and slow compared to him to warrant that. The speed needed to do that is miniscule compared to his actual speed when he's trying.

Yeah, but now he's a bum compared to everyone else in his class, rather than just in general. That's an important distinction.

He'd kill her before she used her domain. Having a domain doesn't magically make you stronger than someone if you can't cast it before they do something about it. Again, you give a toddler a knife than you can easily either take the knife away or get out of range of them. Neither are difficult to do, and the toddler isn't suddenly as strong as an adult just because they have a weapon. Kashimo is the fastest sorcerer of the era, bar none. He can easily just punch her in the mouth to inturrupt the chant, grab her hands and inturrupt the handsign, or just jump out of range and wait it out. She's still weak compared to him, having a weapon that can hurt him doesn't mean anything if she can't use it on him, and to use it she'd have to be stronger, which she isn't, hence why he sees her as a weakling.

This body is FAR stronger than Visions, it will not be as easy to take away as it was then. And MC knows about Thanos in far more detail than Vision ever did, he'd have to fumble hard to allow Thanos to get strong enough to take the stone in the first place.

A toddler can kill you if they have a knife and you're sleeping. That doesn't mean they aren't still weak compared to a full grown adult. She is weak compared to him, thereby she is a weakling in his view. End of story. Extenuating circumstances can let her kill him, but that doesn't make her stronger. If you poison Mike Tyson that doesn't mean you're suddenly a better fighter than him.

I maybe should have read the next paragraph before making my joke.

He didn't. See, he just manipulated the air with his TK in such a way that when you looked at it, your brains automatically filled in the gaps and hallucinated a skeleton construct. Super simple.