The reason why Itachi "followed" sasuke with his stream of ama is because he was purposely wasting chakra, as he wasn't trying to kill sasuke in the first place. We know that amaterasu spawns where the user is looking, & it's extremely fast [
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]. A build up of chakra is needed, which takes time, but once the user builds up enough chakra, he can then use amaterasu by opening his eye. [
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->
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] The jutsu will then stay active until either chakra exhaustion, blindness, or closing the eye again. [
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]
Once an amaterasu user has you in their line of sight & they have ama built up, you get hit, unless you have V2 level speed. The
real trick to countering ama if you don't have the speed is knowing when it will activate which can be done through sensory abilities. [
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] [
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]
Note: Ama may be dependent on how much chakra one builds up based on obito's statement.
So in conclusion for the amount of ama that was used, sasuke should have been obliterated long before the forest caught on fire, he was easy to read as he wasn't moving faster than the naked eye can see. Plus the sharingan allows itachi to
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sasuke's movements, & he was moving in a straight line, there was absolutely no reason why itachi didn't roast him sooner.
You don't need to be proficient at learning a jutsu if it's already extremely powerful by itself. Genjutsu for example, or in extreme cases a bijuudama. If a jutsu is powerful, just knowing how to use it is enough.
As I said earlier they aren't faster than the naked eye so they can't avoid ama if they let itachi charge it up, the only way they can avoid it is if they were to fly so high up that itachi wouldn't be able to see them anymore.
Muu can counter amaterasu, he has sensory abilities plus splitting [but that would result in his defeat]. Muu vs itachi is a good fight but itachi would win more times than not.
Onoki may be able to dodge by making himself lighter but if he wasn't praised for his speed like the raikage then that makes this option unlikely.
If onoki stays stationary he's doomed, Itachi concentrating his amaterasu on a single point would burn a hole straight through the golem & hit him regardless. & it's weight won't change the fact that concentrated ama would burn through it.
While I agree with you, I had to clarify why itachi used ama the way he did against sasuke. He was burning way more energy than necessary & he wasn't even properly aiming for sasuke.