Deidara would probably lose via genjutsu again because he'd probably get cocky having that specialized eye that's supposed to see through Uchiha genjutsu. Deidara was able to nullify Sasuke's genjutsu once, only to fail the second time, which means the eye is flawed and not consistent. There's also no way that eye would be able to handle something like Tsukuyomi, but I don't think Itachi would have to or want to resort to that.
I could see Deidara probably using C2 and gaining high-ground first thing in order to avoid looking at Itachi from eye-level. Itachi also has a disadvantage in this fight due to not having Lightning Style, which would allow him to disable the bombs like Sasuke did in his fight with Deidara. Itachi's use of Fire Style would probably only serve to make the bomb explosions bigger and stronger. However, Itachi can use Amaterasu from a considerable range, so he could probably just concentrate it on Deidara's C2 dragon and blow him up midair, but the fight most likely won't even get to that point. Deidara is powerful as well as intelligent, but he's also overconfident in his abilities, which is what allowed Sasuke to learn the mechanics behind Deidara's techniques and counter them. This is Itachi we're talking about however, not only has he fought Deidara before, his intelligence and genjutsu is far more advanced than Sasuke's, so I think he should be able to take this fairly easily.
