To be fair, this isn't a 1v1, it's a 1v14.
Kaido fought the Scabbards then fought the SNs where the latter group all had ways of getting past Kaido's defenses. It would be hack shit if Luffy fought Kaido alone and inflict this much damage on Kaido alone. But this is a team effort like how Roger and Garp took down Rocks. I do, however, agree that Luffy reaching this strength in 2 weeks is a bit of a stretch.
If Luffy ends up beating Kaido here, he didn't do it alone. He done it with 9 scabbards + 4 other SNs which means that 14 people were needed to bring Kaido down. And this is before considering any possible future events like Hawkins or Drake coming upstairs to assist. Think of it like hunting a monster in Monster Hunter.