Well, 9 Scabbards failed to do it as well as the 5 supernovas bar Asura Zoro who inflicted Kaido a scar.
Same Kaido has yet to show that the scar affects his condition somehow.
Yeah, the Asura slash was not a Gamma Knife.
There are many routes that can be taken to weaken Kaido:
- Yamato has Advanced COC, and so her attacks deal lasting damage.
- Imagine Advanced COC haki beams.
- The Asura attack plays a role later on.
- The accumulated damage from all his previous battles catches up to him.
- Zoro faces Kaido again and does more damage.
There are also many routes to strengthen Luffy:
- New Red Techniques
- New Gatling techniques:
- Red Hawk Gatling
- Red Roc Gatling
- King Kong Gatling
- A new Gear 4th form
- Gear Stacking
- Gear 5
- Awakening
- Marco/Chopper healing to remove his previous damage
- Ice Oni transformation
- Hawkins cards that enable one surpass their limits
Even if we insist that Luffy must solo Kaido, he doesn't need to be able to push Kaido to high diff now to be able to solo him at the end of the arc.
Except this comparison doesn't work because we haven't seen Nami matched Ulti
Nami knocked Ulti down twice.
he even knocked him down multiple times while he was on guard. He could block and dodge.
Pre Udon Luffy knocked Kaido down (and did zero damage). Knocking Kaido down by itself is not really impressive.
Nami vs Ulti is one thing, Luffy vs Kaido is another. The dynamic is completely different.
Kaido's words this chapter confirm that Luffy never truly matched him: