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.
Abilities And Techniques
In the beginning of the series, Zenitsu is seen as a weak person who doesn't seem capable of much until he is asleep. In his sleeping state he awakens an extraordinary power that makes him an extremely powerful and skilled swordsman.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
Zenitsu is an easily frightened demon slayer with a powerful secret. When he sleeps, his inhibitions fade away and he becomes so lighting-fast he can draw, swing, and resheath his sword before a demon's head even hits the ground.
While unconscious, Zenitsu is a force to be reckoned with, but his actual power is discovered when he is awake. While awake, Zenitsu was able to match the upper rank six, Kaigaku. He was also able to keep up with the Demon King, though he was in a weakened state during this battle.
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.
Thunder Clap and Flash—Godspeed is the most powerful of Zenitsu's derivations of the Thunder Breathing first-form technique.
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.
However, in front of Giyu, Zenitsu is a much inferior opponent. He is just a newbie, while Giyu possesses greater strength, immense speed, and superior combat experience. If Giyu fights Zenitsu, the latter will lose miserably.
Among the current generation members, Shinobu and Zenitsu are the fastest characters in the series. According to the Upper Moon 2, Shinobu was the fastest opponent he had faced and was unable to react to her attacks due to its speed.
Sun Breathing has the second highest number of known techniques of all the breathing styles. It has twelve forms that can all be combined into an extremely powerful thirteenth form. It is second only to Moon Breathing which has sixteen techniques, employed by the Upper Rank One demon Kokushibo.
Inosuke is probably the closest to Zenitsu in terms of power, considering they're both new Demon Slayer recruits and they're both less skilled than Tanjiro. If they ever were to face off, however, it's likely Inosuke would win.
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.
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.
How fast is Zenitsu?? I mean it's difficult to tell, but at least 750 mph since he can break the sound barrier.
Kaigaku realizes that Zenitsu is much more versatile and skilled with his swordsmanship than before. However, he still believes that no matter how much Zenitsu trains, he can never defeat an Upper Rank like him.
The Wind Hashira is one of the strongest Hashiras in the Demon Slayer series. He played a significant role in defeating Kokushibo and was able to inflict damage on Muzan. Sanemi's sheer power and speed will be able to overwhelm Nezuko even in her final form.
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. Such a weapon takes immense precision and soundness to use without inflicting personal damage.
Thunder Breathing is one such technique that Zenitsu has used to great effect in the series. Of course, Zenitsu is only the master of the first form, but it goes without saying that the other techniques of Thunder Breathing will also be awe-inspiring to witness in Demon Slayer.
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.
3) Zenitsu Agatsuma
Zenitsu, with his insane strength and lightning speed, can eliminate every Upper-Rank Demon in 1v1 except Upper Moon 1, and Muzan because in terms of strength they are more formidable than him.
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.
The eccentric and flamboyant Tengen Uzui is the current Sound Hashira. This Breathing Technique gives him echolocation and other advanced hearing abilities.