hell of a training is lifting some rocks and training advanced coa for a few days?
Yes, aside from the boost his overall stats received his advanced armament Haki is a massive jump in strength when combined with his own physical strength. Luffy's biggest obstacle was how to get past Kaido's scales, and now he's fully capable of doing so.
if thats all it took to go from neg diff to facing them head on, then we would have a bunch of top tiers running around.
And? this is irrelevant. Luffy got easily manhandled by Crocodile twice in the same arc because he couldn't get past his devil fruit, similar to how his G4 couldn't harm Kaido's body, however after his intense training he already utilized internal destruction, the only possible way for him to harm Kaido and he's "still not ready"? you are trapped into the delusional thought that Luffy shouldn't be close to a Yonko until EoS.
Wake up my man, although Luffy won't solo Kaido, he will deliver the most damage to him and play the major role at his downfall whether we like it or not.
Marines and yonko captured him and couldnt kill him, yet you wanna tell me luffy, who barely learned the basics of this advanced coa is ready?
Once again you're trapped in pre time skip delusion, it was said they couldn't kill him, not couldn't beat him. not a single instance in the manga says Kaido is invincible at combat aside of the usual rumors and heresys.
What do you mean by basics of advanced armament Haki? there's basics for Haki in general which Rayleigh had already told Luffy about, but 2nd level advanced armament only offers one ability (transferring damage into targets) which Luffy is currently proficient at as seen when he evaporated a tree without touching it. there's no more CoA training needed for him later.
Oda has built this arc and Oden's flashback specifically to plan for Kaido's downfall. avenging Oden and the Kozuki family by taking down the Beasts pirates > leads to liberating Wano and opening its border > leads to the Oden prophecy coming true.
Saying Kaido won't fall in Wano is literally like pulling the middle finger to Oda's entire storyline, ignoring everything regarding context/plot.