Well, here's the thing. Deidara has been training to defeat Itachi even programming his eye to counter Itachi's Genjutsu (even though he fell to Sasuke's however, Itachi can't fly to Deidara). Itachi also doesn't have Lightning Style meaning he can't do anything against Deidara's bombs and as for Susanoo, Deidara could just keep throwing bombs at him and would eventually find out about it and eventually, Itachi would get tired out and Deidara would throw a bomb and kill him or something. Deidara, mid diff.

@Ikhimany if it came down to it Itachi has yatsuka beads he called this his most powerful long range ninjutsu. This could definitely one shot deidara out the sky. Deidara also got hit by base sharingan genjutsu so he has no defense for tsukuyomi

Deidara lost to sasuke though and itachi is one of these character that has very op power ups

Even with all his training, Deidara still fell for Sharingan based genjutsu. Tsukuyomi is much stronger than base Sharingan genjutsu, thus he still loses to Itachi

@McGallant Thank you, McGallant.