Angbak from Discord here, providing a slightly more detailed explanation on how this ran went.

First mob pack - proc DR on turn 1, spam posse and stall with white stance, faint devour exalt (weakness and free 3 cost), celeste weakness and Celeste exalt. Stall some and built up exalt (via spamming posse in general)

Boss 
Turn 1 - OE celeste immediately to take the first hit. Retained the 3 0 cost heals to provide heal over time and build up heals on each over time.

Proc DR asap to activate Faint 3 cost double healing. Stall with white stance, weakness and Sanga shielding until I manage to burst boss to 50% health and split.

Post split, all i had to do was to stall the mobs for 6 turns. With Celeste 6 turn exalt cd bring up, I managed to get 1 free turn. 

In conclusion, stall, posse and cheese. OE celeste was critical in this plan which I had from the start of making this acc




Investigation-D-Effect Zone: Wave 1: Alert V Battle Replay
Keeper: Vortice（101678985）

Sanga, Memory Syndrome, The Faraway Eden, Burial Ground's Sighs
Celeste, Gateway of Truth, Merciful Nurturing, Burial Ground's Sighs
Faint, Lullaby Devoured, Cloaked in the Night, Burial Ground's Sighs
Vortice, Dusk and Dawn, Deep Sea's Call, Returnal Line
Drowned Innocence

eee476cf-81b9-3384-aa91-48a600045a8d#E#a

Vortice - 6 active days W1A5 clear