Before Blackbeard we had Whitebeard,Big Mam,Kaido and Shanks as Yonku. If you think about it only Shanks was from the Great Pírate era. The other 3 were in the same era with Roger. It's safe to assume that Shanks was the newest yonku... The question is how Shanks became a yonku? The answer is that he beat a previous Yonku.. Did Roger's apprentice beat him alone? Of course not. The "more safe" way for a supernova to defeat a yonku is to make an alliance. We see how Wano will end. The 3 captains will take the thrones from Kaido and Linlin after their alliance.
To sum up if Shanks defeated a Yonku with alliance the best candidate would be Mihawk. Even though they fought each other before, they are like buddies. Mihawk easily knew where Shanks was to inform him about the miraculous kid(Luffy)Shanks talked about BEFORE . (Another one chat between these two which we don't fully know)They drank each other. At Marineford Mihawk refused to fight Akagami.. All of these events clearly show us that they are not simply respectful enemies but buddies who spent much time each other. My bet is that they formed an alliance and beat a previous yonku. After their win , they fought each other just like what will happen against Luffy and Kidd(or Law)..
To sum up if Shanks defeated a Yonku with alliance the best candidate would be Mihawk. Even though they fought each other before, they are like buddies. Mihawk easily knew where Shanks was to inform him about the miraculous kid(Luffy)Shanks talked about BEFORE . (Another one chat between these two which we don't fully know)They drank each other. At Marineford Mihawk refused to fight Akagami.. All of these events clearly show us that they are not simply respectful enemies but buddies who spent much time each other. My bet is that they formed an alliance and beat a previous yonku. After their win , they fought each other just like what will happen against Luffy and Kidd(or Law)..