Sanji will bake another cake to take out big mom. However this time, it won't just be a normal cake. Orochi, who by this time will be defeated and captured, will be put inside of the cake so that a when big mom eats it, she will eat Orochi as well.
Orochi will be eaten alive and die. However, due to Big Mom's abnormal body, she actually won't die immediately. Rather, she will at first be KO'd. Later it will be revealed that she didn't actually survive eating 2 DFs, rather she was just able to prolong her death due to her body. She will die in Elbaf after making peace with the giants, her family, and the SHs.
Also, Im not speculating that BM won't get fights. In her right mind, BM likely wouldn't accept a cake made by her enemies. Therefore, the allies will have to wear down BM and make her fight for so long that she goes into a hunger pang, at which she will be fed the orochi-cake and be taken out