What happened in the battle between Shanks and The King of Beasts
From the wiki,


  When Kaido tried to stop Whitebeard from going to Marineford, Shanks interfered and clashed with him in order to allow the other Yonko to rescue Ace. It is not known what happened during their encounter, but Shanks arrived at Marineford unharmed. Could this testament prove that Shanks is way stronger than Kaido? No one can say for sure since only Oda knows how it happened exactly, unless it will be revealed in a later chapter. But the fact that he arrived unharmed could imply it was more of a one-sided fight that was probably not worth mentioning.