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I'd put Vishnu in the back row. The front row has a higher chance of being targeted and you want Vishnu to build charges to her skills uninterrupted. Most vets aim for slot 4. I'd also rotate Sati into the main group. Her light resist down (-10%) stacks with Diana's A-frame (-15%) defense down and also with sniper shot (-20%) B-frame down. This can get you to -45% Def down (almost to -50% game limit) and should increase your output damage well.
You should be working towards either Mordred or D'Art at the moment. Mordred will give you Black Propaganda and is a fairly good beginner soul to work with. I ran her for a long time. You do want to get D'Art as mentioned earlier, but Mordred also has a -10% B-frame defense down that will help with content. She should be your main soul as you work towards either Hercules or Shingen. I'd recommend Hercules as your team is somewhat fast, but will be fragile. Hercules is a durable soul that won't slow you down either.
I'd save the regalia for Herc's Axe.
You can put the staff anywhere as long as you have six staffs with you. I'd still aim to have all other weapons be assault and the UE staffs (pride/ascension) aren't bad. You can also take Crowley's Perdu Lab or Cagliostrio's Medic Potion in your EX slot on your soul to keep healing if you switch to Mordred (or Hercules), but they are single target heals (not group).
Dejnov.