Jiraiya knows of Itachi's genjutsu (as he told Naruto about it), so he would be prepared. He would quickly enter Hermit Mode. Itachi would fight him and find out the strenght advantage is clearly in Jiraiya's favour. Then, he would summon Susanoo. Jiraiya would as an answer summon Gamabunta. Of course, Gamabunta would die. Itachi is protected from all sides, so what can Jiraiya do? Attack from below and kill the unprepared Itachi with Swamp of Hell.
Truely an advanced match, as both have advanced powers. It could go either way.
Originally posted by Bentley
I don't know if summoning is a legal tactic in a straight fight due to one on one rule. If Jiraiya cannot get help from the elder frogs he would not be able to enter in hermit mode.
Summoning jutsu is just an attack like any other jutsu. The attack is living, but it's still just a part of the ninja fights. Some fights were fought solely on summons, like Third Hokage vs. Orochimaru.
Originally posted by Bentley
Jiraya cannot use genjutsu by himself, he would probably go down with black fire, Sasuke survived that one only by using one of Orochimaru's best techniques.
But Jiraiya is prepared for Itachi's genjutsu. So it wouldn't really matter. It would all come down to openings in their final tactics, where I believe Swamp of Hell would counter Susanoo.