Just from what I know of you'll do more dmg wtih any elemental mystry/attack ninja's since if I recall correctly 9 tails is weak vs all elements. So using ninja's like Mei an the like would generally be decent. I might be incorrect though.

Yes, 9 tails is weak to all forms of elemental ninjutsu or attacks. As for ninjas and summons, use a summon that combos off chase, use a barrier ninja (anko, shisui, onoki for fire main) and other ninjas that provide buffs like iruka, asuma wind blade, guy, tobirama, hashirama, and so on.

I would recommend getting Yamato's passive skillbroken. It hives your whole team a 50% buff to damage vs 9 tails. I'd recommend using F main, Iruka, Yamato and WB Asuma. I'd recommend replacing WB Asuma with Onoki when you get him.