WARNING: The following contains spoilers for One Piece Episode 915, "Destructive! One Shot, One Kill - Thunder Bagua!"
Kaido, the primary antagonist of One Piece's Wano Arc, has made his presence known. The last of the mighty Four Emperors to appear in the series, he might very well be the most powerful of them all -- or at least the most dangerous.
Kaido rules in alliance with Orochi, the Shogun of Wano, who stole the land after burning Oden Castle down. But until the past few episodes, Kaido has only been resting in his castle, drinking and brooding. Viewers had no idea of the scale of his incredible power until he, in a few minutes, defeated -- and possibly killed -- the Straw Hat Pirates and friends, before beating Luffy into submission.
The Four Emperors
The Four Emperors essentially reign supreme among the high seas of One Piece: Shanks, Charlotte Linlin, Marshall D. Teach and Kaido. Teach stole his position after slaying Whitebeard, meaning that, with the exception of Shanks, all of these guys are not exactly friends with the Straw Hat Pirates.
Kaido in particular has a grudge against Luffy and the gang. He depended on a supply of artificial Devil Fruits in order to transform his pirates into beast-men. These fruits came from Doflamingo, whom Luffy and crew very effectively defeated. Kaido was Doflamingo's biggest customer, who was then cut off from arguably his most necessary resource.
While Luffy didn't defeat Kaido in combat or shake his position of power, he did inconvenience the mighty emperor. Of course, Kaido has the ability to transform into a gigantic dragon, which certainly gives him an edge over Luffy. More than that, however, Kaido can annihilate the Straw Hat Pirates, which he basically does ... while drunk.
Kaido's Drunken, Mountain-Melting Rage
Kaido manages to one-shot all of the Straw Pirates while drunk. After drinking enough, he leaves his base to go on a stroll through the clouds, and ends up attacking Oden Castle, where several of the Straw Hat Pirates are hidden. All he does is rear back and deliver a single blast -- one mighty rush of fire -- that melts the castle.
As of now, it's unknown who, if anyone, survived in the anime. As far as Luffy knows, his entire crew -- save for Zoro, Nico, Usopp and Franky -- is dead. On top of that, Tama, the girl Luffy saved at the cost of exposing himself to Kaido, was hurt.
The two-episode brawl consists primarily of Luffy delivering blow after blow, and none of them hurting Kaido. Yet, the fight ends with Kaido deciding to strike back. All it takes is a single strike to put Luffy down. But more than that, he doesn't only defeat Luffy, but he also attacks his pride. He dismisses his dream to be King of the Pirates, further proving that Kaido doesn't simply win on the physical level but also on an emoti9onal one.
How Can Kaido Be Beaten?
Considering that he defeated Luffy while drunk, it appears that Kaido may very well be unstoppable. One Piece has featured powerful antagonists before; every villain, to an extent, is the most dangerous one Luffy has confronted to that point. Furthermore, he has faced the other Emperors before, but Kaido may be the most dangerous.
Kin'emon's plan to defeat Kaido involved getting him, Orochi and the rest of the crew drunk at a certain festival, and slaying them while they're intoxicated. That might work out for the lower-ranking crew members, but against Kaido, their superior?
Kaido may be the single strongest character in the entire series to date. It seems almost absurd that Luffy can have any chance of victory, unless he can figure out a way to defeat Kaido from within his ranks.
Slight Spoiler for Things to Come
Episode 915 adapts the start of Chapter 924 of One Piece. Knowing that, we can piece together a few details about where the anime will go in the next episode: Kaido didn't actually kill any of the Straw Hats, of course; they survived a mountain-melting blast. We also know that Kaido kidnaps Luffy, convinced he defeated all of the Straw Hat Pirates for good.
Luffy's usual, direct approach will inevitably fail against Kaido. The only solution is the more covert, clever approach that Kin'emon outlined in previous episodes. It's the only way to overthrow Kaido. Luffy clearly vowed to defeat the Four Emperors a long while ago, but beating them and shaking their authority over the pirating world might be far more difficult than Luffy anticipated. After all, Kaido far surpasses Luffy as a fighter. His only hope will be to exercise control and strategy, and to utilize the capabilities of his entire crew.
Or, perhaps, with Luffy out of the picture, the others can better serve Kin'emon's plot to defeat Kaido. It would be necessary for them to avoid a direct fight, because, as demonstrated here, a drunken Kaido can still beat all of them in a matter of seconds. Luffy has never faced an enemy as unstoppable as the raging ocean. In order to win, they will need to use all the strategy and resources available to them in order to win. It will become necessary for them to all work together as a team if they have a prayer of winning.