New Info: Using Ranger with blast arrows, and actually using 4 periapts for strength (I know, makes a lot of sense right?) will let you down the boss, both forms, in less than 10 seconds. I've tested this. Still, the Assassin is far more efficient and Gale Harness insures a faster run, especially now that I know you can use 4 periapts. It doesn't matter to me now though because I have everything, but for any newcomers keep all of this new info in mind.
This is the easiest way to gain the Bitterblack Treasures for Weapon and Armor. Novelty is an option but it's not as good as getting Gear. You must search and play elsewhere for Gear items. At the boss treasure room, make sure that both chests beside the chest that always contains a Moonbeam Gem have Weapon and Armor Lv.3 or you must kill yourself and try again. The chest opposite of those three will either have rift crystals or Novelty. I think there might be Gear Lv.3, but I am not sure.
There were a few stutters in the run to the boss, namely the jump and the Sparyard of Scant Mercy jump. You can slide down a branch there safely but I didn't make it in this video.
The boss takes more damage when he gets stunned or stumbled in some way, and attacking the head does extra damage. Tewi was not set up to use Grand Ingle which would have helped a little more but the AI is almost wholly unreliable in this boss fight, meaning you must be strong to carry more than three times your weight in combat. All in all, it's a 10 minute process that guarantees you good loot. Sometimes the Daimon will drop a Lv.3 bitterblack item, however it is rare. It is either Weapon or Armor in my experience.
Augments used:
Acuity
Clout
Sinew
Vehemence
Opportunism (Secret Augment - Increased damage when clinging to enemies)
Ferocity
It is not the ideal set of augments, but it matters very little, and often times I will encounter enemies I do want to fight (Sirens, Death) for materials. All in all, it is a general use set up that will efficiently maintain you the whole way.
BGM Used:
Lind Erebros - Fingolfin
東方(Touhou) Project - Lunatic Eyes; Invisible Full Moon (Reisen's Theme)