Luffy didn't really beat Doflamingo. It was the ground.
Kid and Law just couldn't hurt Big Mom... Meanwhile Big Mom
How is that "obvious" when she's literally sucking the life out of people when she's at death's door? You realize she already took some life from some randos at the end?
Or is Big Mom sucking the lives of people when she's about to bite the dust fair game in your "2 v. 1 arena" style?