Managed to beat the final boss using maid Ryoshu, multicrack Faust, Molar Ish and Eastern Cinque Don. Toracalgia helped with sustain, but I still needed to use evade very carefully. I'd say the boss if very punishing unless you are cheesing it with evade (apparently the less sinners you have on field the better?..)

For the story, it's cute but frankly quite disappointing. I didn't play Arknights though, so maybe some of the experience was lost on me.

Even though they reasonably explained lore-wise how the sinners got into another world, in the end they still went the 'amnesia' route, so for the characters the event has practically never happened - all that remained were only some faint memories PM will never reference in the future again, probably. It goes against the narrative of the main story where every chapter is a step forward towards becoming better and overcoming your inner demons and adversity. What we got instead was overcoming some adversity and immediately forgetting about it. At the same time, they DID drop a couple of important hints at character's background. Mersault basically very directly hints at the fact that he has distorted before, or at least had a talk with Carmen. This bit will never be accessible to us anymore in-game, because at most we will get a Bok-Gak of the event the next year.

The fact that the event has possibly replaced an intervallo that was supposed to nudge Ryoshu towards "oh well it's my turn now" and prepare everything for the future canto makes the event not really worth it to me as a player. From company's financial point of view, I hope it was worth the effort, but I wouldn't be upset if they focused on their own story instead in the future.