Here's what worked for me.

What you need:
>max ascended herc w/mlb princess ce
>lvl 1 leonidas
>lvl 1 georgios
>max ascended cu alter support w/mlb princess ce
>atlas mystic code

-Put herc in backline.
-Give leo and geo any CEs that'll give you an extra edge, like 2030.
-Don't use taunts on first turn.
-Use mystic code to clear debuff when cu gets poisoned then use his evade and use the cooldown reduction mystic code skill on him.
-Go full unga bunga.
-Defeat Jack, Artoria, and Amakusa in that order.
-Shave off Nero's lives while damaging Ozy when he has his defense buffs down, careful not to kill Nero. Get her to as close to death as possible.
-Defeat Ozy, Medea, and then Nero in that order.
-Abuse resets for everything. Don't use any surviving/protection skills (except cu's evade) unless cu or herc dies, then reset and use em.