This particular point was brought up before...
From whatever we have seen from Tsunade says she is not dat gud in battle. Yeah she beat hell of Oro, but he was already weak. But since she was given the title of Sannin, there must be something to her more than awesome medical techs.
Now Pain:-I will say he is the only in series who was string enough to defeat Itachi. Madara is not considered here. But since Pain used only few of his bodies against SageMode Jiraiya but mangae to hold on and win, without using God Realm gives us an idea that even if Tsunade wuld have been invloved Pain would not have much of trouble.