Metallica's Stand can allow him to freely manipulate iron in the air and ground using magnetism.
He can also use the magnetism to create razor blades, needles or scissors from enemy's within because of iron contains in blood.
I have a theory that Kidd will use this kind of ability to pierce Kaido from the inside out. Blood have iron in it. Just like how Metallica creates razor blades out of Doppio's head (the pink hair guy above).
OT: The only way for the Supernova to win is that they have to get rid of Big Mom somehow, maybe she would go somewhere else. My prediction is that she will fight Marco again since she's now prepared with souls. And they need a bunch of strong fighters to assist them out against Kaido. My final battle prediction is that every supernovas that is currently in Wano right now + Sanji against Kaido which should fulfilled the Toki's Nine Shadows prophecy.