Equipment: Harmonia, Ivory Grace, Any weapon that gives you quick mobility with gift transformation, Mia as a partner (it's viable without a partner, but much less dps and a bit more damage taken)
Passive Gifts: Swift Destruction, Vitality Up, Increased Gift Speed, and Vow of Ichor/Increased Maximum Ichor
Active Gifts: Shifting Hollows, Gift Prowess, Dancing Blaze, Ember Reversal, Iron Will, Supernatural Blood, Cleansing Light, Blood Awakening
Ivory Grace is the highest dark gift stat with the lowest weight that I've found, so I've been using that. The weapon doesn't really matter as long as you have quick mobility and the gift transformation. This let's us make use of swift destruction for the extra 20% damage.
The Harmonia blood code gives us access to Supernatural Blood, which will allow us to use health in place of missing ichor to active gifts. Swift destruction and increased gift speed are self explanatory. We take Vitality Up to be able to make use of Cleansing Light, which will be explained later, and Vow of Ichor or Increased Max Ichor to have enough Ichor to cast both Blood Awakening and Supernatural Blood. The health cost of Supernatural Blood can be cheesed by using Cleansing Light, which heals damage taken over time. Iron Will also reduces the damage taken from Supernatural Blood, and Blood Awakening lowers the cost of the abilities we will spam: Ember Reversal and Dancing Blaze
Ember Reversal is the main spamming ability, as it has the highest damage per cast for its cooldown, Dancing Blaze helps us fill in the gap between Ember Reversal spams with its 3 homing (bounces towards enemies) balls, leading to maybe the second highest possible damage for its ichor cost. Shifting Hollows helps us move fast or dodge enemy attacks with its insanely low cooldown and 1 ichor cost.
The gift preparation should go as follows: Blood Awakening > Supernatural Blood > Cleansing Light/Iron Will > Gift Prowess > 1 Regen
From then on, it's a race against time to spam your abilities enough to kill the boss before Supernatural Blood runs out. Make sure to watch your health and heal when you're low so you can spam more abilities, and always watch the boss to evade attacks.