RosannaHas Favorite Item

Her Favorite Item made her the key Burst 1 Nikke for boss fights against Water Code enemies (= Electric advantage).
Her Burst applies a 30-second Damage Taken increase to Water Code targets and her own damage is on par with a Burst 3 DPS, so she appeared in every top record in Electric-advantage Solo Raids.
She is also used in Arena as a Burst 1 that targets two Attacker enemies, but she has no Burst cooldown reduction, so you need another Burst 1 alongside her.

She keeps landing normal attacks on a target to build ATK and Elemental Advantage damage, then uses her Burst to deal heavy damage and a Damage Taken increase to two Attacker enemies. When a target appears, she gains a lasting Elemental Advantage Attack Damage increase, and each time she lands 500 normal attacks on the target, Frenzy's Increased ATK stacks up to 10 times and lasts 30 seconds. When an ally is incapacitated, she gains Frenzy, fills Burst Gauge, and deals damage to Attacker enemies.

Her Burst Skill, Vendetta, deals Assalto damage to 2 Attacker enemies first, with extra damage if Rosanna is in Stealth. If the Assalto target is a Water Code target, it takes a Damage Taken increase for 30 seconds, raising the whole squad's firepower. Stealth applies at the start of battle and on every 120 normal attack hits, and on 10 normal attack hits she removes the buffs of the 2 enemies with the highest final ATK once per battle.

Running her on auto is fine by default.
Rosanna's Burst has no Burst cooldown reduction, so set the order yourself so she alternates Burst 1 with another Burst 1 such as Liter.

In Arena she goes into offensive lineups that take out two Attacker enemies first with her Burst.

For Machine Gun reload, prioritize the Resilience Cube. Support sustained fire with the Bastion Cube and ammo with the Wingman Cube. If Harmony Cube levels differ a lot, use the higher-level cube ahead of the ranking.

The Damage Taken increase and Assalto damage are on her Burst, and Frenzy's Increased ATK is on S2, so raise those two skills first for Raids. To use the buff removal in Arena, raise S1 to 7.

Used as Burst 1 in Solo Raid boss fights against Water Code enemies (= Electric advantage). In Arena she goes into offensive lineups that target Attacker enemies.

Though a Burst 1, she puts out self-damage on par with a Burst 3 DPS.
Her Burst applies a 30-second Damage Taken increase to Water Code targets, boosting squad firepower.
In Arena she prioritizes two Attacker enemies and removes enemy buffs.

She has no Burst cooldown reduction, so you need another Burst 1 alongside her.
The Damage Taken increase only applies to Water Code targets.
Frenzy's Increased ATK only stacks after landing 500 normal attacks on a target, so she is weak early in battle.