Where do I start...? Yea, the animated battle scene with Akutagawa and Atsushi working together was awesome (particularly when they combined their powers), but I felt disappointed at the same time because honestly...the manga left a much greater impact on me for that part. In the manga, they called the combo attack the "Black Tiger", and though it was definitely well done in the anime, like the rest of this season (minus the first couple episodes), it feels like it was lacking something; it didn't carry the same emphasis and impact.
I think that's not so much on the animation team, but the music. There was far too much BGM in this episode, and half the time the music was too loud compared to the dialogue. That moment with the two in the quicksand was such an eerie, insightful, and shocking moment in the manga...but was overshadowed by the music which made it hard to hear their dialogue.
It felt like the music and the dialogue were competing with one another.
Also, the classical music did not fit the chase/fighting at all. I know that some anime (and even TV shows and movies) use classical music during high-stakes scenes (typically horrific scenes), but it didn't work with this episode. Instead, it felt like it detracted from the chase scene and the first part of their battle.
The music also detracted from the strategic battle going on between Fyodor and Dazai. While some characters (like Ango) spoke loudly which made it over the music, other characters (such as Dazai) often speak in a low tone which sort of just blended in with the music.
I hate to keep saying "the manga" and referencing it, but since I read the manga long before this season was animated...I had certain expectations for this arc. I don't think this was the strongest season in BSD, not by a long shot (that, I'll give to the second season/Guild arc). After re-watching the first couple seasons and then coming to this episode, I also feel like some of the lines and character designs/animations are a little thinner and the quality decreased somewhat.
Most of this will probably also go into the review I'm going to write for this season, but I feel like the impact of the House of Rats arc was just not there in the anime between the slight rushjob, odd angles/character animations, and especially the almost nonstop music (most of which didn't fit what was happening) in this final episode.
Don't get me wrong...I love BSD. But I expected much more from this arc, and it felt like we got a watered down version. The Holmes vs Moriarty/"The Final Problem" feel of Dazai and Fyodor battling in strategy didn't feel as strong in the anime as it did in the manga, and the team-up battle with Akutagawa and Atsushi (and Atsushi's reveal about the Headmaster) felt watered down...mostly due to the music. If the music had at least stopped playing for that chat of theirs, then it would have captured the goosebump-giving feel that I got from that scene in the manga.
So yea...very disappointed in the production of this season.