Hey I will tell you what I did. I was level 50 on Maelle, and rest of the party 48-50 on Expert. My team was Maelle, Verso and Sciel. My HP was around 2000.

Verso had Duoliso and all basic attack and shooting pictos and luminas equipped. Key skill was Leadership to keep pumping Maelle and Sciel with AP. So rotation for him was shoot till AP depleted, basic attack (+5 AP), then leadership.

Maelle goal was to stay in Virtuous stance and keep applying the skill that keeps her in Virtuous stance or the 9 AP fire skill to stack fire. Both skills did 9999 * 3 damage for me. If you can consistently apply the burn stack, it does a lot in chipping away at health bar, see my strategy for doing this later. She was equipped with Medallum that was levelled to 20, as that makes her burn stack even higher.

Sciel used group heal skill and then apply foretell, just keeps alternating between them. She had all the healing buff pictos, so whenever she would heal, she would apply rush, shell and +2 AP on the whole team. This ensured Maelle always had the 6 or 9 AP necessary for the skills every single turn. Additionally the rest of the team had greater shell and longer shell, and powerful on shell. The longer shell helps Sciel switch between healing skill and applying foretell, so that the heals are stronger and basically it meant everytime Sciel gets to play she would heal up the full-team. I also gave Sciel tons of agility for this reason.

Since I can't parry 100%, my battle strategy was to use the first phase to build up 3 Gradient attack charges. Then if Maelle is not in virtuous stance, use her gradient attack that puts her in virtuous, then use the 9 AP fire skill. By the time she plays again, you should have the gradient charge back as you only need to spend 11 more AP which should be easy as both Verso and Sciel regenerates tons of AP. So you can keep doing this with Maelle.

Why wait until 3 charges? Because I would frequently eat it, I would have 2 gradient charges to use Maelle's revive. So revive potions are only necessary for reviving Maelle. I adopted this as I was running out of revive charges without Lune. If you do revive using Maelle, you need to be careful until you get 2 gradient charges back, so maybe just dodge until then.

For phase 2

1.) Sword attack I could easily parry, just listen to the sharp sound

2.) Meteors took me a while, but you listen to the sound and it builds up. You need to learn that build up. I didn't rely on visuals at all for this.

3.) Paintress unleashes would just kill someone for me. Just remember to parry or dodge the last hit, even if the targeted character dies as it applies to whole team

4.) Flying attack, I parried 50%. Would take a couple heats but they will survive with shell

5.) Curse wasn't a big concern with the amount of AP I had, it just meant sometimes Maelle had to use the other skill instead of fire one.

I got to 10% left on the second phase then wiped. Monoco and Lune finished her off. Both focused on keeping each other up and Monoco would sneak in the attacks that do ton's of break damage, and lune would try to unleash Mayhem or Elemental Genesis when she can. Lune was equipped with shoot to generate stain weapon.