Marciana: Marine StudyLimited

An Iron Burst 3 DPS who only pulls her weight in boss fights against Electric Code enemies (= Iron advantage).
Her single-target damage is lower than fellow Iron DPS Cinderella: Crystal Wave and Raven, so even in Iron-advantage Raids she is used as the DPS of your 3rd or 4th squad rather than the main DPS of your 1st squad.
In Arena the Rapture count condition is not met, so the Whistle AoE damage does not trigger, making her hard to use.

She starts battle with Whistle Increased ATK at 5 stacks. While 6 or more Raptures remain, she spends one Whistle every 1 second to deal extra damage to all enemies, and once 3 or fewer Raptures remain for 5 seconds, Whistles build up again. So when mobs pour out, AoE extra damage lands at the cost of ATK, and when fighting a lone boss, Increased ATK stays at max. When 5 or fewer Raptures remain, each Rapture appearing or falling also grants her a lasting Elemental Advantage Attack Damage increase.

Most of her damage comes from Emergency Whistle1's Watch Target. When Full Burst starts, if Marciana used the preceding Burst 3 stage, she deals heavy extra damage to the enemy with the highest final max HP and lowers its ATK, and if the target falls, she marks a random enemy again. Her Burst Skill grants her Elemental Advantage Attack Damage and an Attack Damage increase, and applies High-Risk Target DEF reduction to Electric Code enemies only. Enemies marked as High-Risk Targets take extra damage every 20 normal attacks.

Running her on auto is fine by default.
Watch Target only triggers when Marciana used the preceding Burst 3 stage, so in squads with two or more Burst 3 Nikkes, set the order so her Burst is not skipped.

Marciana has no effect that fills Burst Gauge, so put a Nikke who provides Burst generation in the same squad.

Her reload is fairly fast, so use the Resilience Cube first; the Bastion Cube also works if you have plenty of ammo options.

Her reload is short, so 1 or 2 ammo lines are enough. Fill the remaining slots with Critical Rate and Critical Damage.

Watch Target extra damage is on S1, and Whistle Increased ATK and AoE extra damage are on S2, so raise both skills first. The Burst is a self-buff and DEF reduction against Electric Code enemies, so bring it up last.

Used in Solo Raid and Union Raid boss fights against Electric Code enemies (= Iron advantage). In the Elysion Tribe Tower, bring her when you need an Iron DPS.

When fighting a lone boss, she keeps the Whistle Increased ATK at max.
Her own ATK and Elemental Advantage buffs are large, so she has less need for separate ATK buffers.
When mobs pour out during a boss fight, she deals extra damage to all enemies.

She has no effect that fills Burst Gauge, and as an Assault Rifle her own gauge generation is low.
Her Burst debuff and bonus damage against High-Risk Targets only apply to Electric Code enemies, so her damage drops sharply in other-element boss fights.
How the Watch Target passes to another enemy is not consistent.

A limited summer Nikke.