I took 12 up originally. 3 from each element. A +30% leader, an AoE, a booster from each. With third skills unlocked things actually get easier. But you have to keep the warriors at the same level or things will get much harder if one or two are far ahead.
My first 12 were. Zahra, Ping, Utu. Zuma, Hector, Mohinder. Mungo, Joan, Kwan. Ironheart, Doongara, Badda. After that I did Tiberius, Kofi, Kuro.