If Deku wasn't the main character, but still had One For All or...?If Deku weren't the main character, Tokoyami, Sero, Iida, Minetta and Invisagirl could realistically take him. I agree the the three could take the majority of the class. But those fights now among the top 10 would be closer than you think.
And let's be honest, Shinso won their first bout.
Froppy vs Baks at sports festival goes to Baks easy, but those two one on one fighting where they were last arc she could pull some tricks.
Without One For All and its vestiges, yes, Deku would have lost to Shinso. However, anyone who's fighting Shinso one-on-one can beat him if they know how his quirk works and are even being the least bit rational. It's literally just not replying to him. Unfortunately for our dear protagonist, he's an emotional person which is just the type Shinso can exploit.
On a related note... Shinso should take notes from Monoma for the mind games.