Grand finale help, please?

Hi!

For Grand Finale, having Mash and possibly even George to manage the enemy is incredibly useful. In addition to this, a strong burst damage unit than can take down Nero is handy, considering her layered Guts. Raikou could work for this, and she has the added bonus of an extremely powerful AOE. Jack's debuffs are harsh, so taking her out is paramount. In the end, I think the team I would personally use would be:

Mash - very, very good here, with her taunt and teamwide buffs.
Herc - he is very good for bursting down Amakusa without an Avenger, as well as being handy against every class. The issue is keeping him safe: but remember, even though his Guts CE and skills are very useful, Ozymandias casts Sure Hit on the party after he dies. Make sure to save him for near-last, or at least after Amakusa is gone.
George - he takes reduced damage from Medea, and so he makes a good tank as long as you can get Jack off the field fast. My recommendation would be to have him in the very front of your backline, and put a strong Caster or Berserker in the front, like Raikou or a support. Remember: Jack has Buff Nullification for three turns, so do not rely on having buffs immediately.
To take care of Nero and generally wait her out, David is the best choice. He does decent damage and can stay alive for a very, very long time. You can use him to protect a secondary damage dealer, like Rider Kintoki to handle Medea once it's all said and done.

Happy questing!

You have a ton of options, tbh. You could just fill up your roster with any 6 of those servants, and keep throwing yourself at it and it would eventually work out.

But what I used was lvl 35 Georgios + 70 Hans + 90 Cu Alter (with White Rose MLB) on my front line, and Jack, Chloe, and Anne and Mary Archer (also with White Rose MLB) in my back line. Chloe was just there to take up space, and I would have rather used Mash or some other type of support in her place, but my Mash is stuck at level 30 until I can do more story mode, and I don't really have any other supports.

I used the Atlas Mystic code to remove the buff blocker from Cu, and then I used Hans' buffs and Georgios' 2nd skill to expend the buff blocks on Georgios so that he could use his taunt.

We killed Jack first and then Amakusa. By that time, Georgios died, so we were able to bring out my Jack. We then killed Altria, and then my Jack removed Ozzy's buffs and then Cu wrecked him with a NP.

At that point, preotection from Arrows and other evades weren't useful anymore, so it would have been great if I'd had Mash available to buy Cu a few more turns of life. In the end, it wound up with Anne and Mary making a last stand and finishing everything up. I killed Nero and then Medea.

You could basically replicate my team, but just use Mash where I had Chloe, and then use Chloe or Herc with a white rose CE in your final slot.

You've got so many more servants than me though, so you might have even better options.