Since the beginning, all the strongest fighters in the alliance goes all-out and attack Kaidou. No one can even touch him due to Dragon Force: Kaidou's permanently-active CoA that he obsessively trained and hardened for decades since infant, acting as a body armor, millions of times stronger than Bari2 No Mi. The reason for his title, World's Strongest Creature. No one can reduce his lifeless boredom, emptiness, no one can force him to move an iota from his standing ground. Until he senses Luffy's presence. Kaidou orders his subordinate to make way for the boy, and get away from Wano Country itself, to the seas, as far as they can. He also tells them that if they haven't finish the Strawhat Alliance by the time he's finished with Luffy, he'll kill them all himself.
With that order, the entire Beast and BM Pirates and alliance pirates sail to the ocean and continue the war as naval battles before boil down into group fights and 1v1s. Marco vs King aerial fight (flashy fireworks in the sky), Sanji and Law vs Queen with tricky sea battles and large scale attacks and object manipulations (ships flying and seakings flying everywhere teleported from and to the sea).
Kyoshiro and Zoro vs Orochi (whose Hydra form is actually very strong and resilient and also able to swim in the sea, like equal to King, and also very cunning), an underwater battle between two swordsmen and sea monster, seawaves as high as mountains, one hydra head being cut but regenerated, rinse and repeat, but Zoro manages to awaken demon spirit Takemikazuchi as the Lightning Sword Deity and cut all nine heads at the same split second), then both of them recruit Yamato.
---
Yamato will face BM after Luffy vs Kaidou is over. Strawhat and 9 Scabbards vs Flying 6 and Numbers etc. Many casualties, many deaths from Heart Pirates, Kidd Pirates, Scabbards and Minks, many betrayals and backstabbings and double-agents etc. If we want some more gutpunch, Chopper, Nami and Robin die after being tortured by Beast Pirates and CPO Agents. Replaced by Law (who is left alone in Wano since all of his crews are dead, so he enters the SH crews as a surgeon), Jimbei (can both steer and navigate, and also had inherit the incomplete map from Nami), and Yamato (who is both extremely strong and also knows poneglyph language and mystic arts since Roger or Shimotsuki Ushimaru or whichever Wano expert taught her in the past).
---
The only ones left in Onigashima are BM, Kaidou, Luffy. Kaidou questions BM if she wanna die as well since she doesn't leave. BM says she just wants to watch since it's too risky for her to interfere in a battle between two guys that are seemingly dead equal as two WSC's. Kaidou says he recognizes BM's intention to kill whoever is left, and hog the glory for herself. But he cannot simply ignore the ultimate threat that is standing before him, Monkey D. Luffy. The boy already has Poseidon as his ally, Kaidou can't let him take his daughter Yamato just because her capabilities in Poneglyph reading and mystic arts.
During all this time, Luffy just stands perfectly still, his eyes closed and his mind for the billionth time simulating the fusion between Devil Fruit's Neural Awakening and Skysplitter Conqueror's Coat.
The boy opens his eyes, BM floats in the sky with Hera's cloud then she says "Begin."
Both Kaidou and Luffy leap forward meeting at the center, the first clash alone splitting the sky and disintegrate the giant skull at the same time.
---
Something like that.