Only way for Luffy to win against Big Mom in Wano after he wins against Kaido is my theory of Law and Kidd's teamwork destroying her heart to pieces come true, imo.
We know the Yonkos have insane endurance, they can function even without vital organs. If Kidd and Law go down to BM, but with BM becoming a literal heartless, a fight between Luffy vs Big Mom can still happen since Luffy really only needs to resort to a battle of attrition until Linlin runs out of lifespan.