I said I was going to show this off so here it is.
I've greatly refined it to more or less make Excellus a joke.
Bring in Itsuki/Touma/Yashiro. Touma, who will be your lowest leveled character, should be 40+. Any lower makes the fight extremely risky, and you don't want that for a long fight like Dark Yashiro.
All three should have HP Fitness. Endless Act isn't required but helpful. Touma needs Javelin Rain, Life Gain, and Gaia's Whisper (Side Story 2). Itsuki needs to be your healer and buffer (buy Rakukaja Stones and Attack Mirrors and have plenty of healing items), he'll pretty much never be attacking at all due to counter.
Yashiro pretty much counters the entire way through. It's silly, but it's the fastest way he's dealing damage.
Use Gaia's Whisper while Dark Yashiro is whole, and when he splits, use Javelin Rain. Build up to 2 SP again and wait for him to Quad-split (happens at 1/4th HP). Use Attack Mirrors with Itsuki liberally at that point and make sure everyone stays alive, and close out the fight.
Edit: Note that, at least on Hard Mode, I don't see any other way this can be done at least in a timely manner.
Seeing some new game runs completed has inspired me to work on this game. I came across o-den's notes http://seesaawiki.jp/dsoctxt/d/RTA%A5%C1%A5%E3%A1%BC%A5%C8%A1%CA%B8%B8%B1%C6%B0%DB%CA%B9%CF%BF%A1%CB which shouldn't be hard to translate, so I'll post that here when I finish. Does anyone else have route notes (even if they're unfinished) that weren't posted in this thread?
Just realised this forum existed >>
So here is the route that I've worked on, based on HeroMystic works and his inputs: https://pastebin.com/fCTMPHcN
Some things might get some slight changes. also at the bottom is listed all the weapons we get and their requirements.
TL;DR: the route uses Itsuki/Tsubasa/Ellie until Yashiro joins, where he is switched with Ellie, staying behind for Mass Destruction/Repels/Open Audition purpose. It skips using Touma entirely after chapter 2, as Itsuki is good enough to deal damage on Dark Yashiro, still uses Kiria for bosses to Lullaby/Labyrinth, and never uses Mamori. Also cuts out a lot of performa grind for weapons that are unnecessary.
I still feel like the route is improvable in a few places but at this point I'd need newer eyes. The Dark Yashiro fight especially is very wacky and I feel could be done better.
The current route that has been made can go sub 9 hours realistically. Any lower relies on rng. A lot of the fights are pretty volitaile especially from ch 3 to ch 4 and that's too many fights to confidently say sub 8:30 is reliable.
The two major general improvements that need to be done is:
-The RNG hell from ch3 to ch4. -The grinding sessions in ch 5 and ch 6.
Thank you for posting the route, this answers a lot of my questions. What I've been focusing on is trying to understand how leveling up, stage ranks, and carnage mastery work. I was able to setup homebrew to look at the actual numbers in memory. Stage rank is strange, I think the best way to farm it is to cast party buff spells like rakukaja which gives 4 points plus a point to the other 2 party members in the battle, compared to session attacks which only gives each participant 1 point. Maybe information like this could optimize the grinding sections.
