Alright, sicnce I'm an OP, I will dubunk it
First, I think Pisan is underestimate Nagisa so much by repeatly said "he's just a 15 year old" again and again, by he forgot that Nagisa is a 15 years old guy who was trained by the greatest assassin of the world and the top member of goverment who specialize in combats. Not just a mere 15 yesrs old boy.
Second, Nagisa can reading/discernment people. He can just look on Yuno and tell that "She is really strong and dangerous" and can decide about best way to deal with her.
Third, from second, what choice that he decide to deal with her, fight her face to face? No way, he should know that he can't win agiainst her with that. Nagisa specialize in assasination, so his decision should be escape from rooftop, find the place to hide in building, hide his bloodlust and for a change for a surpise attack that can finish her before she can react, like he do when he defeated 4 classmates in a row.
Forth, Nagisa used to defeated several people who stronger than him. Such as Takaoka, an assassin who use whip, Karma and several trained militaries. Especially for last one which I pretty sure that they stronger and have experience on combat more than Yuno. So Nagisa weaker strength is not the point. Nagisa's quick mind did allow him to win against a tougher opponent (who is also not a slouch in the smarts department either).
Fifth, Class E's special cloths was designed to have great resistance with protect wearers in several situation. And Nagisa can survive from ruthless attack by
God of Death who is far stronger than basline 9-B (Whick I pretty sure that he far stronger than Yuno too). So I don't think one stab from Yuno can take down Nagisa. And when she do, It will let Nagisa a change to use assassination technique to finish her.
Sixth, Of cause Nagisa Clap Stunner should work on her, Yuno isn't Karma who can negate it by bite his own tongue. She don't know about this technique and don't know about how it work, so Clap Stunner + Stungun is enough to finish her.
And this is my argument about why Nagisa is the winner. So I don't see how the result of match should be changed.