PhantomHas Favorite Item

With her Favorite Item, she became the next Water main DPS after Snow White: Heavy Arms and Dorothy: Serendipity in boss fights against Fire Code enemies (= Water advantage).
Even with just one boss, her self damage rises greatly, and her Burst also applies Damage Taken increase to Fire Code targets, so in Water-weak Solo Raids she was widely used for top records in Maid distributed damage squads with Bready.
She is also a main DPS in Elysion Tower, but is not used in the Arena.

She repeats a loop of stacking Thief's Dagger up to 3 stacks with normal attacks, then consuming them at max stacks for heavy damage and distributed damage increase. Each hit on a Rapture without a Calling Card stacks Calling Card DEF reduction and Dagger, and landing 30 normal attack hits on the target also stacks Dagger, so max stacks cycle even with just one boss. Hitting an enemy with a Calling Card raises the attack damage of the next 1 shot.

When Dagger reaches max stacks, she deals extra damage to Calling Card targets, deals distributed damage to all enemies, and stacks distributed damage increase on herself up to 3 stacks. These stacks disappear when she uses her Burst. Every 10 normal attacks, she briefly gains Increased ATK and distributed damage increase. Her Burst Skill deals distributed damage to all enemies, applies Damage Taken increase to Fire Code targets for 30 seconds, and raises her own Max Ammo for 10 seconds.

Auto is enough.
Simply landing normal attacks on the target stacks Thief's Dagger, so there is no need to split aim across multiple enemies.

For an Assault Rifle, prioritize Resilience Cube for reload speed and support ammo recovery with Bastion Cube. If Harmony Cube levels differ greatly, use the higher-level cube over the ranking.

After Elemental Advantage damage and ATK, secure Max Ammo, and add 1 line of Hit Rate if accuracy is lacking.

Enemy-wide distributed damage and Damage Taken increase are on her Burst, so raise it first. AoE distributed damage and distributed damage increase at max Dagger stacks are on S2, and Dagger stacking and Calling Card are on S1, so finish by raising both to 10.

Used as a main DPS in Solo Raid boss fights against Fire Code enemies (= Water advantage). Also a main DPS in Elysion Tower.

Even with just one boss, Thief's Dagger reaches max stacks, giving her high self damage.
Her Burst applies Damage Taken increase to Fire Code targets for 30 seconds, boosting squad damage.
In Maid distributed damage squads, she takes one of the two DPS slots alongside Bready.

Her Damage Taken increase only applies to Fire Code targets, so her value drops outside Water advantage boss fights.
Her damage only rises greatly after raising her Favorite Item to phase 3.
As an Assault Rifle with low Burst Gauge generation, she is not used in the Arena.