With full intel, Itachi would first start off by trying to take out the Naraka Path as he knows that takes away Nagato's ability to restore the Paths. He'd need a relatively quick attack to do this, so I'd assume Itachi would use an Amaterasu to take that out first despite the toll it takes on him; however, Nagato has been shown to sense the chakra build-up for Amaterasu. Amaterasu would render the Naraka Path useless before the Preta Path could get there to absorb the flames.
The Animal Path's multiple summonings would prove difficult for Itachi. Jiraiya stated that the summons were used as a means to confuse an opponent and to hide his location [
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]. Itachi could keep track of the Paths/summons with the Sharingan’s precognition and ability to see chakra, though the location also plays a role. He could easily take them out with Susano'o via Totsuka/Yasaka Magatama or Amaterasu, but again the chakra tax of these techniques plays a role on how he would use them. Itachi could also try to find a way around the summons and destroy the Animal Path, thus dispersing the summons. Itachi's crows in a swarm may be able to distract the summons temporarily or he could use Crow Clones or Kage Bunshins for feints.
Itachi has better CQC than the Pain Paths as the Paths had trouble against SM Naruto [
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,
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,
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], and Itachi matched KCM Naruto in Taijutsu; the most trouble he would have is against Deva or fighting multiple paths at a time. Itachi was able to dodge Bee's seven sword attacks with only the 3T Sharingan, so fighting off hand-to-hand attacks from multiple directions is well within Itachi's capability. Sasuke was able to hit Itachi with a windmill shuriken in the leg during their battle due to his deteriorating vision. If Pain lands a blow with one of the chakra rods, Itachi’s chakra will be disrupted, and he will be bound by the Outer Path giving Pain an opening.
Again, with intel, Itachi won't needlessly use Katon or Suiton techniques as he knows they can be easily absorbed by the Preta Path. He'll only use ninjutsu as a distraction or if the Preta Path is taken down.
Itachi will try to stay out of the way of Human Path as much as possible. A Bansho Tenin + Human Path combo could prove deadly, but seeing how fast Itachi was able to summon a Susano'o arm to block Sasuke's own [
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], he could use BT's pull to his advantage and take out either the Human Path waiting to grab him or the Deva casting the technique. As seen with his fight against Kakashi, Pain sacrificed the Asura Path to save the Deva Path, so the same could be said for the Human Path in this scenario.
Itachi's speed would allow him to dodge projectiles from the Asura Path, and perhaps attempt to use an Exploding Clone to destroy it.
Itachi would have the biggest trouble against the Deva Path. Itachi would surely use Susano'o to defend against the higher powered ST considering one was able to KO all three of Naruto's boss summons at once. Pain would use Chibaku Tensei to end the fight if it goes this far. Considering that he wouldn't have had to expend as much chakra as he did in the fight with SM Naruto + Konoha + 4/6 Tailed Naruto, Pain would be able to make a larger Chibaku Tensei. The only problem is that Nagato has to have Deva come within a certain range of his real body's location in order to utilize CT [
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]. Otherwise, Itachi isn't destroying a stronger CT by himself as he doesn't have the benefits of being an Edo-Tensei and will be quite exhausted from the fight up till this point.
Contrary to belief, the Rinnegan is not immune to visual genjutsu. That was just someone spreading false translations. Jiraiya was able to use a paralysis genjutsu on three of the Paths at once [
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]. This genjutsu, however, is effective within the area of a barrier, thus not giving a chance for it to be broken in either of the normal ways while the barrier is up. That's not the case with Itachi's genjutsu. Nagato, who has massive reserves of chakra can identify which of the Paths is under genjutsu, and with the Rinnegan’s linked vision, can disrupt the genjutsu with a surge of chakra or by shutting off the chakra transmission to that specific path and reactivating it; this leaves an opening for an attack by Itachi.
All in all, if Pain stays back for the fight, he can wear down Itachi with his superior chakra reserves and finish him off.
