Alright First Just because Hidan Said That He was the least Skilled And Slowest Member Of the akatsuki Does not make it true .. And even if it did everyone in the akatsuki is extremly fast and has a high speed rating in the data book ( except maybe zetsu and konan .. have not checked) So being the slowest of the group does not make hidan Slow in general.
Also Hidan Only Having A slight Chance To Get a Hit off ??
You must be registered for see links
How is tsunade going to get a punch off when being thrown At with that... In order for her to even throw a punch And hit hidan she would have to be caught by the scythe which giving Tsunade's Healing powers is not a big deal... BUT that one Injury is enough to get hidan The blood He needs And seeing how one punch from tsunade would not Break Hidan
You must be registered for see links
Because he has clearly Shown he can handle Pain that would make a normal Man Drop.. So the punch From tsunade would not finish him .. And he would be able to retreat to his diagram ( which he draws BEFORE battle) and once in there tsunade cant do anything .. If She Tries To Hit hidan while he is in the diagram she will be hit as well...
Katsuyu
Katsuyu May be the worst offensive Summon there is what did she do During the sannin fight besides get caught by manda.. Nothing And her verry few attacks are Slow and weak.
You must be registered for see links
hidan has evasive Feats however And was able to attack even while evading the quite fast shadow stitching Jutsu .... So not only should he be able to evade her attacks but he could even throw his scythe in the process Aiming it at tsunade whether she be on the ground or on katsuyu ....
Also regards To your Tsunade breaking the diagram argument.. She has no idea what it does so she wouldnt bother to break it .. and if she tried to after hidan got the blood she would hit hidan in the process = Hitting herself as well
*edit shikamaru is a better Analyst then Tsunade is .. no doubt