Jiraiya IS capable of beating Itachi, and why does everyone forget that Kisame admitted himself that he was not able to fight against Jiraiya, or was he also trying to "spare" Jiraiya's life during that encounter? The way I see it, Kisame is the reason why Itachi is going to loose this fight. Kisame can't win against Oro or Jiraiya, to this day we still don't know how to kill Oro besides sealing him like Itachi did, so we really can't speculate on how Kisame is going to do it.
Itachi's downfall in this fight is his incredibly low stamina, against a Sannin with huge chakra reserves and stamina, plus virtually limitless chakra in SM, and another Sannin who is incredibly hard to kill and was only defeated because he was sealed, this battle will not be the quick and easy kind like Itachi likes it, he will be extremely exhausted, and using Susanoo from the beginning will also take a toll on him. Sword of Totsuka is not that hard to dodge, Jiraiya has speed feats in SM capable of dodging Amaterasu and so on and so on. The only thing that will be a challenge in this fight is genjutsu, and Oro would have finished Kisame by then to help take Itachi out.