1 Can't Beat: Rui Although Zenitsu managed to overcome Rui's elder brother, it's unlikely he'd be able to do the same with Rui. Given that Rui is one of the Twelve Demon Moons, he's an incredibly powerful creature — one that almost kills Tanjiro, who's more capable than Zenitsu is when it comes to dispatching demons.
They would have died. Zenitsu almost got killed by the brother. And Inosuke plus Tanjiro were struggling with the father.
Muzan even went as far as to expect Rui to be able to defeat a Hashira by himself despite him being a Lower Rank. It was also stated that if Rui recovered all of the power he shared with his "family", he could have been a good match for the Hashira, and possibly could have been stronger than them.
Demon Slayers are known for their strength and bravery in battle, but not Zenitsu. Despite his cowardice, he's one of the strongest in the series.
3) Zenitsu Agatsuma
If he could, Rengoku would win simply because Zenitsu cannot perform Godspeed more than twice. Since Zenitsu only knows the first form, Rengoku would know what to expect and accordingly counter his moves, taking him down in the end.
Zenitsu is clearly on the same level as Hashira. If we compare him to Shinobu, his speed is far better, while she lacks quite a lot in raw power. There is a good chance that he could beat Shinobu if the two decide to fight.
But, comparing the two characters in their best forms, Zenitsu might be just a tad bit faster owing to his Godspeed move.
As further credence to his strength, Zenitsu was able to advance to the highest point of the Hashira Training. Zenitsu singlehandedly killing an Upper Rank demon.
Why Zenitsu could not become a Hashira in Demon Slayer. Fans have wondered whether Zenitsu ever got to realize his dream of becoming a Hashira at all. Unfortunately, the answer to this question would be, no, he did not. Zenitsu, along with Tanjiro and Inosuke got way stronger than average corps members with training.
Zenitsu mastered the first form to its full capacity but failed to learn the rest. However, the first form itself is extremely difficult. With his master gone, Zenitsu can no longer learn the remaining five forms. Therefore, during his fight with Kaigaku, an Upper Moon Six, he created a 7th form.
No they would not the fight happened, in the manga and the Hashira needed “extensive” outside help to beat Muzan Rengoku Muichiro and Tengen weren't there because two are dead, and one retired but the 9 Hashira by themselves cannot win.
1. Gyomei Himejima. Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
After his treachery, Kaigaku later becomes Zenitsu's archenemy. His cowardice would later lead him to his downfall when Zenitsu overcomes his insecurities as Kaigaku mocks Jigoro. Kaigaku's Thunder Breathing training would lead to his Blood Demon Arts being related to lightning.
Judging by that encounter, Nezuko is the weaker demon between she and Rui. She certainly can't defeat him on her own, and it's questionable if she can even do so with help.
1 He Was The Only Lower Moon Killed By A Hashira
It makes you wonder why he was rated as lowly as he was, as you could argue, he was stronger than Enmu.
Shinobu Kocho is a capable Demon Slayer whose worthy of being the Insect Hashira. That being said, she's arguably the weakest Hashira when it comes to pure strength.
Shinobu Kocho Is the Weakest Hashira
The Insect Hashira, Shinobu's petite frame prevents her from being physically able to decapitate any demon, be it an Upper-Rank or the weakest demon in Japan. At one point or another in the manga, all nine Hashira in Demon Slayer encountered an Upper-Rank.
This mastery allowed Zenitsu to form a Seventh Form of Thunder Breathing that is unique only to him: Honoikazuchi no Kami. While this move might seem similar to Thunder Breathing's First Form, the speed and power of this technique are on a whole new level.
How fast is Zenitsu?? I mean it's difficult to tell, but at least 750 mph since he can break the sound barrier.
To sum it up, it is possible for Zenitsu to fight without fainting and it has been shown clearly in the manga. While he only perfected the first form, he trained hard enough to create a new form that didn't exist before.
Tanjiro isn't Faster Than Zenitsu, not at all. He is Strong but Slow. So, Zenitsu Will Probably Chop Him Down to bits before tanjiro even pulls out his sword properly. There ya go!
Hinokami Kagura, also known as Sun Breathing, is easily one of the strongest Breathing Styles in the series by a wide margin.
1) Shinobu Kocho
The Insect Pillar of Demon Slayer Corps, Shinobu is undeniably the smartest individual. She might not be physically stronger than others, but her intelligence is what kept her going.