I didn't "provide proof," because the outcome is frightfully obvious and the only people who would demand "proof" are no doubt Itachi fanboys who could not be convinced even if Kishi himself said their hero would not win....
Allow me to waste my time anyway

-- Itachi's best techniques and what sets him apart from other shinobi are of course, his eye techniques with the sharingan. Most of these are genjutsu and that is no doubt the field he likes to play in and where his greatest advantage lies.
Pain is immune to such techniques. The rinnegan is partly responsible, but most important is how pain completely counters ALL Itachi's eye techniques simply by being 6 in 1. Itachi cannot lock 6 people at once in a Tsukuyomi, even if he catches one, the other 5 are on him. The 6 bodies also share sight, meaning the body Itachi is trapping can look away but the other 5 can keep looking at him and tell the 6th what he is doing. Pain would be hell for Itachi, he is built completely and entirely to counter a ninja like Itachi because of his 6 paths jutsu. Remember Kakashi's whole speech about, "don't fight a genjutsu user 1v1, run away, they have the advantage. But fight them in a group of 2v1 or more and you have it." Well how about 6v1? I thought so.There would be no one worse for Itachi to fight.
So Itachi can not use his bread and butter of eye techniques. Well let's be honest, after genjutsu, Itachi is pretty basic with 2 special moves thrown in and that's about it. Pain of course does not even use much genjutsu--if any--so now that genjutsu is out of the picture i think the outcome is all that much more obvious...but of course some fans disagree. =)
So basically this is where the hilarity starts. Obviously Itachi is not going to take on 6 s-class ninja in hand to hand combat and win(lol) so it basically becomes the 6 paths of pain and their countless insane jutsu vs....amaterasu and susanno....wow. Maybe you see why some people can't take fanboys opinions seriously?
So somehow you have a ninja with 2 techniques beating 6 ninja with inumerable techniques? Ok sure, that seems fine. However again, amaterasu should be out of the picture, pain can absorb it, push it back or simply 5v1 Itachi while he "aims" it at one of the bodies; again, pains numbers make such a technique purely laughable when looked at objectively.
So this is where the itachi fans find their final harbor.....Susanno. Now of course, Susanno could simply be trapped by Chibaku tensei, could be absorbed by fat man, or avoided till Itachi kills himself.
I have to ask...are you serious? Do you actually put your feelings aside when arguing this? Because quite frankly, realistically, this fight comes down to 6 paths of pain and all their Jutsu vs Itachi and one justu. it's just stupid, I'm sorry, I like Itachi aswell, but the argument is "silly."
I said it before, I'll say it again: pain is the worst opponent Itachi could possibly face in this manga so far. 1v1 Itachi would give anyone hell, even ninja like madara and the 4th, I truly believe that. But because of how pain fights, he simply counters Itachi's main techniques completely. This is just a no win situation.