In terms of damage for 9 Tails or Group Summons, Fire main is the only main character to use if you want to rank for damage. Generally you would use Iruka, WB Asuma, and some sort of barrier ninja. Depends on who you have. Hashirama is preferable but if you don't have him, you could use Shisui, Mei, Danzo, or Onoki for auto barrier, or even just use Anko. Keep her barrier up the whole time, rather th*ing the 2nd part of the mystery. You just have to keep her in the first position and put up her barrier before she attacks. Iruka and WB Asuma are fairly easy to get. WB has frag drops in a lot of place but esp. in Survival Trial. Iruka you can get by simply turning in your extra character frags after you get them to 5*.

Stacking so many high % buffs means way more damage than multiple weaker attacks. And fire main has a 50% and 40%. Iruka and Asuma are 30%'s and Barrier is 30%.