Didn't Itachi already prove how he can beat a Sannin with two moves at an age of 13 years?
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Tsunade hasn't the feats to break through Itachi's 3T Jutsu that easily. No matter who it was in the past, even if Orochimaru, who possesses high intelligence and a full Genjutsu score, had no chance against the Genjutsu of Itachi's basic Sharingan. Itachi is always quick to act and ends the battle within seconds, as proved always. It was a matter of seconds, when Itachi caught Orochimaru in a Genjutsu and cut off his arm. In Tsunade's case it would be the neck. As Exaar mentioned, Itachi possesses not only paralyzing Genjutsu, but also illusionary, one would have great difficulties in seeing through - seen by Deidara, who almost activated the explosion on himself.
Be certain, that Itachi could end the battle before Tsunade would even summon Katsuyu, activate Creation Rebirth or Byakugo. That's how a battle with Itachi always ends. The moment it starts, the victim is possibly caught within a Genjutsu without realizing it.
At this point, one should pay attention to Itachi's speed.
He got behind Bee without latter realizing [
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]. Combining this speed with his usual feints and powerful Genjutsu, Itachi would quickly set a deadly blow at Tsunade, probably even while latter is still in base. So even if Tsunade would have the means to successfully break Itachi's illusion, Itachi would only need to maintain the Genjutsu for a few second, until he blitzes her.
Katsuyu is a summon, which in fact falls for Genjutsu like Manda did to Sasuke's, who didn't even have Itachi's level of Genjutsu at this time.
Itachi's skills are on such a high level, that it becomes irrelevant to bring all of Tsunade's abilities into discussion, because Itachi never allowed his victims to go all out, ending the battle in the first sequence. That applied to an Akatsuki like Deidara, and also to a Sannin like Orochimaru. At a mere age of 13.
Kunai strike at a humans weak point (neck, throat) or the rest of a humans weaponry (explosive tags, set at a weak point again), explosive Kagebunshin which explode right before here in close combat, etc. would show considerable effect on her if she has not activated her Byakugo before (and doesn't need to be the case if Itachi has the first turn with Genjutsu).
Or to make it simple, to not even start arguing with medical Ninjutsu, Itachi will strike off one of her hands, disabling her from forming any hand seals required for her medical Jutsu and regeneration (for example seen by creation rebirth). Knowing Tsunade's abilities, Itachi's intellectual level would allow him to act quick.
As for Genjutsu, Itachi has also several ways to cast one, whether with his eyes, his fingers or his crows [
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]; latter while distracting the victim at the same time [
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], making him an easy target for the upcoming Genjutsu.