Yeah, I wouldn't bother with Dragon God's Breath anymore, except for targeting CLD weakness. The damage should be roughly similar to Explosive Flame Sword now. Samidare Slash is still worse outside of MP cost.
Regarding your run against
, I think part of your damage issues is a lack of additional multipliers like Herbalist's Herb of Awakening or Strategist's Furious Scheme. With those two in play, you can increase your damage by a little over 50% (except for Patchouli due to Grand Incantation), accounting for the lowered buff decay rates too. If you're running the new update, Strategist will properly reduce damage now too. As far as I can tell,
does not utilize any status effects whatsoever, so you can adjust your gear around that to get more durability and/or damage as necessary. As you probably noticed, some of your low SPI characters suffered enormous damage from Start of Heavenly Demise. I think 400+ SPI affinity and 450K HP will be safe provided you have a MT healer, which you unfortunately do not. I don't think you can really do anything about that except make some changes to your party composition or apply a bunch of ATK/MAG debuffs, SIL, whatever you can.
So specific party adjustments would probably be
-make Komachi an Herbalist for Placebo Effect + Herb of Awakening. Placebo Effect has the same 7000 delay as a Direct Attack, so whenever you'd have a dead turn and you can't or don't need to apply Herb of Awakening or switch a character, you can use Placebo Effect on a character to increase their buffs. This way, none of Komachi's turns are wasted.
-make some buffing characters Strategists. Reducing the buff decay rate is even more valuable than enhanced buffs, and you get the +10% damage increase/reduction skills to go along with it. Since you do a lot of switching around, you'll need multiple Strategists, so pick whoever you think doesn't need subclass effects. I'd recommend 3 of Iku, Yukari, Renko, and Ran, with the remaining character being an Enhancer or Herbalist (I'd say Herbalist for Iku and Enhancer for the rest), but if you find yourself needing less than use fewer. Ideally, I think 1 Strategist that you keep on the frontline the whole fight is best, if you can manage that.
-make Aya an Enhancer. This is more to give her something to do with her subclass. If she doesn't need to buff anyone's SPD or use Divine Grandson's Advent, she can give an ATK/MAG buff. Alternatively, you could use Elementalist, but I think you need better buffing ability than an additional damage boost when it comes to Aya. If you find yourself not using Enhancer buffs because your characters already have high/max ATK/MAG buffs, or the delay on Enhancer buffs is too high, then go with Elementalist instead. Elementalist buffs have 8000 delay, making them really convenient for Aya since she can act again quickly.
-Adjust Yuugi, Suika, and Flandre's equipment and library levels so that they have 400+ SPI affinity (use Affinity Boost on Flandre and Suika), and get rid of Patchouli's Earth Armor in favor of a third Tupsimati or anything that increases MAG and HP. If you can get Patchouli (and everyone else) above 400-450K HP, that should suffice. If that doesn't do it, further increase their HP and affinity. Also, make Yuugi and Suika Transcendents to give them more HP and damage reduction, this should be more useful than Warrior. Transcendent also has higher base ATK and an unconditional 10% damage increase, so it should have a bit more damage output as well.
-The enemy version of Start of Heavenly Demise is a composite attack that uses both ATK and MAG, so if you can debuff these, that would help immensely. The boss also appears to be very susceptible to SIL and TRR for further damage reduction.
As a reference, this is my run for
:
https://www.youtube.com/watch?v=0E_ZCNYq2Sg. Character setups (for the ones that were actually used) were
-Aya: Elementalist w/ Tokugawa Statue & 3x Long Sword "Ringil", max SPD for level bonuses
-Iku: Strategist w/ Tokugawa Statue & Regalia/Medicine of Life/Long Sword "Ringil", max SPD for level bonuses
-Akyuu: Herbalist w/ same setup as Iku
-Tenshi: Murakumo's Blessing w/ Tokugawa Statue & Magic Sword Chaos/2x Scourge, max ATK for level bonuses
-Rumia: Enhancer w/ Tokugawa Statue & 2x Regalia/Medicine of Life, half HP and half MAG for level bonuses
-Miko: Dragon God's Power w/ Tokugawa Statue & 3x Machine God Lucifer, max MAG for level bonuses
I don't think it's very helpful to figuring out how to fight the boss with your own party composition though, as it relies on things that you don't have, like some passive damage multipliers (Rumia's Human racial bonus, Akyuu's 16% elemental weakness bonus, Miko's 8-16% buff increase), a MT healer (Rumia again), and Instant Attack to refresh ATB rapidly. I'd rather not recommend changing your party composition unless you actually want to do so. Hopefully the above changes should make the boss more doable.
--
Oh yeah, unrelated, but I realized I overlooked that
did receive a buff, and now grants a +999% ATK increase instead of +800%.