Demon Slayer is a "monster hunter" shonen anime series whose combat system is based on an arms race between flesh-eating monsters and the trained experts who fight them. Overall, the demons have the advantage as supernatural and immortal monsters of the night, but they do have a few key weaknesses that demon slayers of all ranks can exploit.
The sun itself is a demon's worst enemy, along with simple decapitation with a Nichirin sword. The demons themselves know that decapitation is a death sentence, so a few of them have adapted and overcome this obvious flaw. This, in turn, challenges the underdog demon slayers to get creative once again.
How the Smartest Demons Overcame Their Last Major Weakness
Aside from their vulnerability to the sun, which not even Muzan can do anything about, demons only have a serious vulnerability to decapitation with Nichirin swords, and they're generally aware of this. Even the strongest demons such as Akaza are careful to not let any demon slayer slice their head off, but most demons don't do much to address this problem. That has allowed Tanjiro Kamado and his friends to slay one demon after another with a single strike to the neck, but a few demons decided to fight back after all. In fact, it's surprising that there aren't more demons doing this, given how they all have the exact same weakness.
Several Demon Slayer villains have found creative ways to make sure decapitation won't be the end. The first example was the massive hand demon, who hugged his sturdy arms around his neck to prevent decapitation, and then the Upper Moons started following suit. In the "Entertainment District" story arc, the himedere demon Daki had her head hacked off, only to burst out crying rather than crumble away. Even Tengen Uzui, a Hashira veteran, was shocked until the truth came out -- that Daki and Gyutaro shared their neck weakness. Only decapitating both of them at the same time would actually kill them, making the fight far more challenging for Tanjiro's team.
Now, Upper Moons like the emotion-based Hantengu and the cruelly artistic Gyokko are likewise circumventing their decapitation weakness, with Hantengu having several bodies at once and Gyokko fighting via proxy bodies. By now, such strategies seem like an obvious step for any demon of sufficient power to take, with the three weakest Upper Moons all making sure that one decapitation won't finish them off. Otherwise, these demons would be disappointingly easy to defeat, and it's possible that Doma and the six-eyed Kokushibo also have countermeasures in place against decapitation. If they don't, they're not creative enough to do so with their blood arts or are strong enough that they don't see the need -- a comment on either their foolish hubris or their vast power.
The Arms Race Between Demons & Slayers
The battle between demons and demon slayers has been raging for a few centuries now, but in Demon Slayer, the arms race appears to be reaching a climax, and one side or the other will likely win soon. Demons and slayers alike are going all-out to finally settle this never-ending war, and if the demons can even find ways to survive decapitation, then the demon slayers must hurry and innovate their own countermeasures or face certain doom. If even the Hashira can't rely on decapitation to finish the job, then it's time for some human ingenuity, and Demon Slayer will deliver.
If cutting off a demon's head isn't a sure thing, demon slayers can use new weapons that don't rely on decapitation, which Shinobu Kocho the insect Hashira is already doing. Shinobu lacks the physical strength to decapitate a demon, so she uses deadly wisteria poison to sting a demon with her slender Nichirin sword and destroy them. If demons like Daki, Hantengu and Gyokko keep hiding their vulnerable necks, then demon slayers may have no choice but to follow Shinobu's example and use poison in missions where powerful demons are expected to appear. It remains to be seen if even Shinobu has synthesized a poison strong enough to kill the Upper Moons, but it's a solid lead to pursue.
Genya Shinazugawa also added a whole new dimension to Demon Slayer's combat system with anti-demon shotguns, allowing him to quickly and safely attack demons at range. Demon Slayer hasn't yet shown an Upper Moon dying to shotgun blasts, but the fact remains that human technology is now advancing too quickly for demons to catch up. Demon Slayer takes place during World War I -- an era where modernization is rapidly reaching new heights, especially in warfare, and storybook monsters like demons can't keep up. If the demon slayer corps can hang on a little longer, they can beat the demons in this bloody arms race and claim final victory, even if the Upper Moons and other demons are now becoming resistant to decapitation.