Regarding the Prisma Event

Salter is a hard hitting Buster damage dealer with a strong AOE NP. Would definitely focus on her. Medea can easily spam her NP for decent damage and will be easier to lvl as a 3*. Lily is only if you really need a dedicated healer, but you already have white dress for that.

Nightengale would be my first recommendation. Not only can she hit hard but she can also provide good support through her healing skill and NP which also removes status effects and she has a 3 turn targetable Buster buff. She’ll be very useful.

Big thing to remember is that even though female servants are buffed, Quick damage is not. So Ushi and Atalante may drop in priority unless you have no other option in that class. AND don’t forget Orion is technically a male servant. Have seen people forgetting that one a lot already.

I would also focus on caster Marie due to her survivability. She can also fill a few more roles than her rider counterpart as she can provide some C Stars and has an AOE damaging NP, although admittedly a weaker one

Again I'll be short.
Level your riders specially Ushi to help you, and Marie to stall around when you can't. Astolfo will also work too BTW. Borrow a drake or a mebd.
Find a lancer level her up or borrow one since you might need one since I heard we're fight Kuro or something not sure.
Try to level Medea Lily since she'll have added damage on her and just to help. Her older version would be nice if she's your only ST caster and some bosses are weak to arts.
Try to level Nightingale to support your support drake or Mebd.
That's all I know sorry about that...