This CoW can be kind of tricky if you don't have some key units. Mainly it's hard to get enough burst DPS while also fulfilling all the other various requirements for staying alive. Most clears I've seen revolve around having either Shrine Maiden Tsukiko for her Light imbue imperil Area Effect or Elegant Temptress Lilith for her double-mitigation LB, not to mention that both are pretty top-tier DDs in CoW regardless, so they make it easy to cap.

What I found was that even with 2B at EX 2 I couldn't easily get enough damage while staying alive the entire fight. (I have A2 at EX 1, which means I can't bring them both to the fight and still enable all the modifiers.)

Here are the things the fight absolutely needs*:

• 75% both physical and general mitigation, 100% uptime, to deal with the Damage over Time. Species-specific mitigations can help here, but there aren't a lot of great AoE options in the units that are practical for this fight.
• 85% ATK/MAG breaks, 100% uptime, for all the damage, but the DoT especially
• Enough occasional healing to keep the DoT from killing your more vulnerable units, keeping in mind the boss reduces your healing the first 3 turns
• A cover tank (or provoker if going the mirage route) with a crapton of DEF, and near-300% lightning and light resist after buffs
  • Esther can also use her shift-magnus to absorb lightning for the team (although this puts her in cover-mode so the rest of the team doesn't matter), also she can absorb lightning all times with Storm Kickers equipped.
• The ability to perfect-dispel the boss's barrier on the same turn you burst at least twice, but preferably more.
  • Keeping in mind that unless using Kaito's Crashing Waves, that unit won't be contributing to DPS.
• Either AoE Light Imbue if you're going with 2B's imperil/buff, or AoE Lightning imbue and a good imperil option if you're going with Sylvie's AoE buff
• Status and breaks resist

* - unless you're more clever and can figure out loopholes to these

At the end of the day, there aren't a lot of combinations that will work for this and leave you with more than 2 full-strength DPS units. Here's what I found:

• Sylvie and Esther's magnus abilities were insufficient to cover mitigations for the entire battle. At the end of the day, I had no choice other than to give up a slot to Runda, whose 80-85% mitigations with easy 100% uptime did the trick.
• Even though Kaito is already jacked for his fight with multiple ways to perfect-dispel the boss, I really wanted to trade him for a stronger DPS. What I found in the end, though, was that I needed him for his ATK/MAG breaks, as I couldn't get 100% uptime on ATK/MAG breaks any other way.
• Without Tsukiko, the only units I had that could take an AoE imbue slot and make it worthwhile were White Dragon Ling and Sage of Thunder Sakura. I did a lot of experiments with Cressnik but the fact he can only imbue once kept him from the final team.

In the end I succeeded with the following party:

• Kaito EX 3 - Breaks, dispel, mirage, uses his magnus to clear the buffs on turn 4, even does some damage. I couldn't do the fight without him.
• Sylvie EX 3 - Her AoE 100% lightning buffs were a big part of making Lightning work. Other than that she's a good utility knife for morale and elemental resists, and her BS LB is a great buff for the burst turn, but I might have replaced her on a Light team.
• Runda EX 3 - Simply essential for his mitigations. I couldn't do the fight without him.
• BB Olive EX 3 - DPS.
• 2B EX 2 - DPS, mirage, AoE killers, backup elemental resist
• ST Sakura EX 2 - Imbue, imperil, area effect.
  • If like me you've got a couple dozen master crowns collecting dust, enhancing her area of effect imperil to 50% is a good way to finally make use of them.

Note that like I said earlier, this team really only has 2 main DPS units (with 2B still at EX 2), but it was enough to get close to 3 million on the main burst, well above the 2.5 million necessary to cap.

I did a 9-turn clear, 6 turns for the initial burst (so that BB Olive could do 2x SLB) and then another 3 turns (for BB Olive's final SLB).

Before that I tinkered around with a lot of other combinations to try and get to 3 DPS. E.g. Sakura can do the perfect dispel, but without Kaito there was nobody who could break the boss sufficiently. Or I tried to swap Sakura out for WD Ling and replace Kaito, but suddenly I had nobody who could perfect dispel. That said, I do think this team likely would've worked with WD Ling in Sakura's place, although it would have a rougher time hitting the cap. Ling is tempting to use as she can help out with morale gain and LB fill, enabling a 5-turn burst instead of 6-turns (as it'd build morale quicker and could fill BB Olive's LB on turn 2 to get her magnus up and running).

Ironically, both 2B and BB Olive self-imbue Light, but with just the two of them it wasn't enough damage, and although I could slot an extra DPS, without an imbuer none of the other heavy DPS options I might have slotted in could do Light. Honestly without Tsukiko's imbue/field effect or unless you have enough EX-levels to get both 2B and A2 onto the field, it's hard to make Light work for this fight. I'm glad I had Sakura at EX 2.

I'll add, morale gain can be tough with this party. Both Sylvie and Runda had Sylvie's VC, and Kaito in his base form had Celestite Rod and Treasured Memorial Ring. Sakura was dual-wielding Conductor's Batons and had Galbana Lillies. Obviously all the other CoW gear was in play as well. Runda even had Calamity Border (but only used it a couple of times). It was enough to get to 0% by turn 6, but not by turn 5.

Finally, I'll say that the AoE mirage approach worked very well for me. First turn, Sakura imbued the team, and Kaito did his normal attack targeting Runda, which dispelled his cover (no damage since he had lightning resist). The boss's AoE moves only occur on his turn-specific actions, and they're well-documented on the wiki. You need to keep track of how many attacks your mirage abilities absorb and how many turns they last, but it's not too hard. I think I started out turn 1 with Sylvie's Paladin's Defense, which got me through the first 4 turns. Then I used 2B's Avoid on turn 5, and Kaito's Receding Tide after the burst. Alternately Runda might have been able to cover-tank the entire thing (he gets the benefit of not being hurt by the boss's human/beast killers), but I'm not rushing back to find out.

Anyway, that's the summary. Good luck to anyone still attempting this!