Go on. But remember, No fish wank allowed here
![Beckmoji :beckmoji: :beckmoji:](https://i.imgur.com/o0TFufq.png)
In terms of powers, ninjutsu will not work—not even katon or Amaterasu. Despite its failings, Samehada can absorb heat based ninjutsu but has a dislike of it. However, Kisame is capable of forcing Samehada to do as he pleases despite itself [3 —>4]. Likewise at any point that Itachi decides to pull out a V3 Susano'o, the minimal requirement needed for Yata Mirror & Totsuka Blade [5], Kisame will have created a wide enough sphere of water and has good odds of escaping any attack or even when unfused [6]. When fused with Samehada, Kisame is faster than a perfect Jinchuuriki in water [7]. Don't forget that Naruto, no documented swimming feats, was blitzing 2 Tomoe Sasuke when influenced by the Kyuubi, in water during VoTe1 [8].
So in short; Amaterasu gets absorbed and the threats of a V3 Susano'o and the Totsuka blade will never come to fruition because by that point in time, Kisame has a lot of liquid & water to move around in.
So Itachi needs to hit Kisame with Tsukuyomi before Kisame fuses with Samehada—because Kisame no longer requires eyesight to hunt when fused [9]. The principles of disrupting regular genjutsu are as follows; irregular chakra flows [10]. Kisame, via Samehada absorbing and sending back chakra can create an irregular flow to avoid regular genjutsu in the same way a Bijuu & Jinchuuriki are immune to regular genjutsu. It's really only Tsukuyomi that gets Kisame.
There's other factors like battle IQ; Which Itachi exceeds Kisame but Kisame is no slouch either, stamina & speed. Itachi has a massive speed advantage which cannot be ignored and he's the more intelligent of the two, so I give it to Itachi 6.5/10 times but kisame has nearly everything required to take down Itachi.