According to all the experimentation I did so far with final boss combinations here:
https://gamefaqs.gamespot.com/boards/234634-saga-scarlet-grace-ambitions/79207187 <- warning: big mess of tl;dr and spoilers.
...you have to do the following to get the hardest version of the final boss in every phase:
Urpina: Chapter 2 Scarlet Shards, choose to destroy the Ibex Ring -> Chapter 3 Phoenix.
I'm not sure if Chapter 2 Sigfrei -> Chapter 3 Scarlet Shards, keep the Ibex Ring gives you the hardest version for all three phases or just one or two.
Balmaint: One of these methods should give you the hardest version for all three phases:
a) Scarlet Shards for either Intermission and Arthur must leave your Party at the start of Part 7
or
b) Earth Serpent for the second Intermission and go through the Urpina Rescue Route (can only be done during Part 6A).
I did a lot of experimentation with Taria and Leonard and haven't got to Balmaint yet with these tests to confirm which method (if not both) works.
Taria: Two ways to get the hardest version through all three phases, as confirmed through my tests:
A) Chapter 2 Earth Serpent -> Chapter 3 Scarlet Shards (doesn't matter what you do in Chapter 3--you can blow up Shards or go straight to Azhuacan after saying Yes to Sasha).
or
B) Chapter 2 Sigfrei -> Chapter 3 Earth Serpent, must go through the Urpina Rescue Route.
Leonard: His methods are a little spoilerific, so I'll be a little vague (similar kind of vagueness in my guides regarding the results of the Antonius and Urpina Kidnapping Events, so this will still be helpful to you without spoiling anything):
To get the hardest version for all three phases, there are two Shard Drowning/Lissa Wish Combinations that work:
(these assume that you gathered every possible Scarlet Shard possible (aside from the possibly-bugged, possibly-intentional second Cerenaif shard, although it's possible that the number of Shards gathered doesn't matter as long as you have more than just the two you started with)
A) Choose to drown all of the Scarlet Shards on the first choice and when prompted for Lissa's wish, choose the second/bottom choice.
or
B) Choose to drown only Leo's Shard for the first phase, choose the second/bottom choice for Lissa's wish, and then on the second phase, choose to drown the woman's Shard.
(edited 3 months ago)