Upgrade a primary weapon you are comfortable with and don’t build around bleed for this fight. Save Dread Axe for the stagger window. The Scarlet Wraithblade reward can produce slowing flowers through combos.
Look for Dread Axe under Phantom Edges, not in the primary weapon list. The Chinese equipment screen labels it 巨凶斧. Footage: 友利奈绪大魔王, Weeping Creek Valley 04 (P5), 18:15.