The Big Mom Pirates go to Marineford to rescue one of their captives.
At Marineford are the same characters present there and they have the same level of involvement. Ex: Mihawk was fighting fodder dudes, Akainu was facing the Yonko.
Big Mom has all of Totlands soldiers/fighters and ships to her disposal.
All of her children capable of fighting are there
Condition of victory is to rescue the captive and escape with Yonko and said captive.
Rd1: BM has the aid of Luffy and the impel down gang that was Present at Marineford(Jinbei, Crocodile, Ivankov, prisoners)
Rd2: BM is still allied with the Germa and get their help