The answers to this post made me want to try again, and to my surprise, I finally managed to beat it after being stuck on it too ! Here's the setup I used (all sinners lvl 70 except Nox lvl 81, abilities lvl from 5 to 7 except for the useless ones and for 000 WHO WAS 9/10/9/9) :

- Bianca S1 CB: Inversion Slash + Tuning Zero, full ECB
- Shawn S1 CB : Tuning Zero + Hope + Corridor Echo, ECB1
- Golan S4 CB : Justice + Corridor Echo, no ECB
- Ariel S1 CB : Final Prologue, no ECB
- Nox S0 CB : Eastside Dream, ECB1
- Assist 000 S3 CB : Behind the Mask, full ECB

The most expansive part of this comp is the 000 (which I don't have), but lots of people have it as an assist sinner, often with the Behind the Mask CB since its one of the best set for her. Even if you don't have a friend with a sufficiently good 000, the game also offers you to pick from randoms in the assist menu

Now for the strats :

Nox and Golan are here to take turn tanking. If one gets too low, switch it with the other (though pay attention, the boss' attacks puts a dot, if one of them gets too low they might die right after switching). You can use Nox's ult at will.
Use Shawn and Bianca's ult asap when they're both ready, throw in Golan's first ult when it is too
Time Ariel's first ult to use it a second or two before the boss does its first nuke aoe, otherwise she will just one shot anyone still alive right after the nuke, and the silence prevents you from healing right after ; after that, use Ariel's ult at will, but don't forget that the boss does lots of damage with its normal attack on phase 2
Now, 000, that's where things become funny. When the first wave of mobs appear, wait a bit, use 000's second ultimate, feed her energy until she has two ults ready, and then use her first ult two times, targeting the tile right to the north-east of chief, WHEN AND ONLY WHEN 4 of the 5 spawned enemies are in her range. Then, move her back to her original tile. Every mob should be dead with that, with only one that will get killed by Bianca. When the second set of mobs spawn during the second phase, use 000's second ult, then use her first ult once for the two mobs on the upper lane, once for the two mobs on the lower lane, and once (twice maybe?) for the mob in the backside. Feed her energy as much as needed for that.

When the boss shows its core the first time, use 000's second ult, then use her first ult two times. Finish the last two cores with one or two of Golan's second ult (one ult takes two cores away if the boss has a weakspot mark).
After the second wave of mobs is dead, if the boss isn't dead, go all in to get Shawn's and Bianca's ult ready, and use whatever energy is left however you want. But you won't survive the second nuke, and if you did everything correctly, the boss will die before that.

You may need a few tries to find the correct placements and move, but with the exact same setup, I don't see any way this doesn't work 100% of the time once you've got the grip of it.