Hakai is the strongest attack displayed so far. Whitebeard tilting Marineford is not an attack. Hakai would've busted Onigashima and Zoro stalled it for couple of seconds, while the others tried to escape.
Could a single person replicate that? Depends on how much effort the 2 Yonko put into it. But it seems like that wasn't 2x 100%. Imagine pumping their attack full of CoA + CoC.
So Primebeard imbuing his Supreme Grade blade with CoA, CoC and Gura could've achieved even more.
I also think that a KKG with barrier + internal destruction CoA + fully mastered CoC would be even more devastating, since it is focused power. A KKG with average CoA broke through spider web + God Thread and propelled Doffy with such force into the ground, that as a result, part of a city was flipped.
Now imagine a KKG amped by barrier + internal destruction + fully mastered CoC right to the body. It would destroy Onigashima too in the process.