That's assuming Shanks simply didn't outsmart him. Let's not forget that a lot of the insanely powerful guys have some serious weakness. Like Luffy being an idiot. Zoro has no sense of direction, Sanji and women….. I can imagine Shanks using his brain to trick the obviously unstable Kaido into somehow taking himself out
Yeah but that was mostly because they are scattered brained idiots. Marco didn't pay attention and got himself handcuffed, Jozu got first both hijacked by Dofla and then later on frozen. If they had paid attention and fought the right people I think they could have at least stood as equal to the Admirals
Emperors are well above commanders and they are above the Admirals as I believe.
I wonder how it happened though.