Itachi isnt so tough at all...he is overrated.
Itachi is a genius as far as ninjutsu and genjutsu...that has been shown.
His taijutsu hasnt really been shown so far...what he did was a piece of cake with a completed Sharingan. He was aware of Gai's Taijutsu abilities and probably knows that Gai has trained to face a Sharingan user (Kakashi).
Things could get too interesting at that point. Which would damage the story-line as it is now.
To beat Itachi i'd say Kage Bunshin might work if used properly.
If you can manage to create a few bunshins which happen to be pretty tough and skilled along with the user (Sandaime). Then perhaps you can get around Tsuykomi.
Also Itachi's abilities are limited where Naruto for example clearly surpasses him in stamina and chakra. Itachi cant fight for long..(count out the jutsu Tsukyomi)
Anyway I doubt Itachi even with help of Kisame would win against both Jiraiya and Tsunade. Tsunade's punch or kick would be sufficient if they hit.
And Jiraiya/Tsunade definetly has/have alot more techniques up his/her sleeve...techniques which made them "legendary" throughout the ninja wars.
To beat a Sharingan User or Byakugan User speed in most cases would be sufficient. As Lee stated before...Sasuke didnt stand a chance. The incident of Naruto vs Sasuke will not happen again. Since Sasuke's Sharingan evolved+his use of the CS.
Once a speed like Gai or Lee has with Gates starts...then it's pretty much over.