Try Oberon-Cybele, with Zephyrus and Caspiel if you feel like having a long match. That group will give you -50% to -90% damage reduction, and -30% Def. Since it's going to be a long fight, Andromeda with Black Propaganda should compliment that.

You should be able to clear the Koihime Ultimate with that team with ease, at least. Actually, Mii used buffs, I think? If so, bring Cass instead of Andromeda.

It's not optimal since your damage output will be low, but it should clear you just about every non-Fire Ultimate. Using Auto-Skills at boss to save you the effort. And well, as you clear Ultimates you'll start to get MLB Eidolons and SSR weapons, and that'll slowly but surely make you stronger.