You do realise ninja can fight without looking at their opponents, right? Especially in Gai's case who is so an expert in CQC that he can notice and counter attacks
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. Amaterasu has even a pressure that any notable ninja should notice. A Itachi standing still to concentrate to use said jutsu will also
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. With Gai high speed movements, Itachi shouldn't be using it
Yes, But guy isn't a sensor. Itachi, contrary to popular belief, is simply too fast.
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he moved an inch, and then roast guy with a
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Sure, guy may notice and avoid some of this, but guy will be evidently over whelmed. The fact that itachi performs all these fire jutsu by holding his
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and his ability to
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will all lead to guy's defeat.
Even when guy enters gates, Crow and shadow clone feints would allow itachi to effortlessly substitute himself
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. ITachi will be sitting in a bush while guy wastes hirudora.
Or, in the very slim chance guy get's the upper hand over him -- through his incredibly speed, hand seal speed, sharingan precognition, Itachi simply resorts to Yata mirror and it's checkmate.
And, this isn't taking into account Seal-based Genjutsu which would trap guy. Guy never showing any remote genjutsu abilities, thus making it unlikely that he'd break it easily.
People like to rush to MS, but the fact is, Itachi is easily top 10 without even resorting to his MS.