How to beat Tales of Onikurenai last boss in one go

[without Onikage Rui or even YJ Rui]

so hey everyone~
only three day remaining for this event, but better later than never, I guess. at the beginning I thought I don't have a chance to beat this tough boss in one go since I don't even have YJ Rui for good tanking, but eventually I found the trick. our main keys will be some 4* that you can get via the ads gacha - Goddess Akira and Musketeer Hikari. Pirate Shizuha will be very helpful, but I think she can be replaced by other middle-row tanks (I hope, though).

The main idea is to create THREE lines of defense before the boss reaches the back row - one is regular tank (we'll use Christmas Karen), one semi-tank semi-damage-dealer (Musketeer Hikari), and one middle-row tank (Pirate Shizuha). we can do it since the boss targets only one stage girl at time, except when climaxing. its climax hit only once, so it's very easy to dodge with an evasion buff, and that's why Musketeer Hikari does great as a second tank, even though she is in the first row.

As for damaging and ACTs prioritizing, we should aim to reach climax with Hikari before the boss beats her, and after that we STOP USING HER. she is here only for tanking and climaxing, not for regular damage dealing.
For main damage dealing, we'll use Akira and Fumi equally, and aim to reach their climaxes by the time the boss beat the middle-row tank. that means that even when the boss climaxes and generates the anti-snow buff - don't mind it. you still attack with them in order to build brilliance. even with the snow-resist buff, they will do more or less the same damage as Hikari anyways.
And yes, this is very important - we WON'T CLIMAX WITH AKIRA/GUEST ONIKAGE RIGHT AWAY! if we do, we'll lose the freezing effect that can be inflicted only once in this battle. it's essential to keep this effect to the end in order to prevent the climax against the back row, and making the most of Akira's climax.
Generally, the boss SHOULD climax three times before we freeze it - one time against every one of the three first tanks. when it reaches the back-row and is ready to climax - that's when we use guest Onikage Rui's climax in order to freeze it, then Akira's and Fumi's. that way we save Fumi from the last climax damage and let her finish it off the next turn.

I know it's sound a bit complicated. well, this battle indeed requires more focus and being careful than usual... I hope the video will make it more clear:

Example Video

Keys:

• Goddess Akira + 15% Effective Element memoir.
  One of your two main damage dealers. we we'll prioritize her and Fumi's ACTs for the most of the battle, especially her 2AP one. her 3AP act deals the same amount of damage as her 2AP, and its only adventages are the meaningless perfect aim buff, and the more brilliance. keep that in mind.
  We'll aim for building her and Fumi's brilliance for the most of the battle, and release the climaxes on the right time - when the boss beats the middle-row tank and is ready to climax against the back-row.
  As for her climax - since we're gonna use Onikage's for freezing, the freezing effect of her climax won't be exploited. nevertheless, her climax is still great since it hits only once. frozen enemy will recieve more damage on the hit that breaks their freeze status, and since Akira's climax hits once, it'll be much more effective than using Onikage's three hits climax after Akira's freezing.
  Edit: There is an alternative for her in case you don't have her - Sun Nation Futaba. since we don't rely on Akira's freezing effect, we can replace her with SN Futaba and still get the more or less the same damage dealing, since Futaba comes with an anti-beast auto skill and one-hit climax too.
• Benikage Fumi + Damage Buffing memoir.
  Our event 3* that come with very nice kit: good damage, great 3AP act, great climax, and the usual event damage buff. she will be our second main damage dealer.
  Her 3AP is amazing. it deals almost as her 2AP, but it attacks group (effective at the start of the battle, when those "diamonds" are still protecting the boss), and inflict Normal Defense DOWN debuff which is just GREAT for the collective damage dealing of this battle. be sure that the boss have this debuff by the time you release Onikage's and Akira's climaxes, it's very important!
  Like Akira, we'll prioritize her ACTs and build her brilliance during the battle, then release her climax on the right time.
• g. Musketeer Hikari + 40 Brilliance memoir
  She'll do both the roles of a tank and a damage dealer in this battle.
  Since this boss attack only once with its climax, she we'll easily avoid its first climax with her initial evasion, and can stall the boss even more with her evasion generating right before the boss is going to climax again. it's essential to climax with her ASAP since she won't hold for very long, and that's why the "Friends at the Aquarium" memoir will be very helpful.
  after climaxing, you can ignore her ACTs completely and prioritize Akira's and Fumi's, or just using her evasion generating 2AP if you don't recieve any other good ACTs.
• Christmas Karen | Water Goddess Michiru + 2 Fortitudes Memoir.
  Our regular tank. they are here only to take damage and nothing more. don't use their ACTs unless you really don't have any other ACT to put (and no, don't try to reach climax with Michiru to freeze the boss. it's risky and not efficient at all).
  This is the place to say that if you have YJ Rui, you should use her as your tank. my friend has her and she is really the best tank you can use in this battle. the only adventage of Karen and Michiru is that everyone should have them (well, at least Michiru).
• Pirate Queen Shizuha + 2 Evasions Memoir.
  Okay, this is somewhat tricky. the idea is to use you highest normal defense middle-row stage girl. it happens to be her in my case, and she's do great with her ability to generate evasions (even though you should use her 3AP very carefully). she is here for stalling the boss even more while the back-row attacks and builds brilliance. I hope that even if you don't have her, your highest NDEF middle-row stage girl will be suffice.
• Guest: Onikage Rui. for damage buff and right-timed freezing climax.
• Note: all stage girls should be at least rank 7 and fully potential bloomed.

Well, that's it. I really haven't believed that this boss is doable in one go without a proper tank like YJ Rui, but I'm glad I found the way to do it with nothing more than ads gacha 4* girls, and I hope it'll be helpful for you too!
Good luck everyone! ^~^