Helm: AquamarineLimited

Helm: Aquamarine is an Iron AR DPS who can apply both Burst cooldown reduction from the B2 slot and Electric debuffs.
In Electric boss fights, damage taken stacks and her Burst gains extra damage, so she shines.
Where Electric enemies do not appear, or in PvP, other options are easier.

Landing 30 normal attacks deals extra damage to the target, and when Full Burst Time starts she reduces all allies' Burst Skill Cooldown based on activation count. The same effect stacks multiple times, speeding up the squad's Burst rotation.

During combat she suppresses 1 random enemy, and if that target counts as Electric, damage taken increase stacks up to 5 times for 5 seconds. Her Burst deals distributed damage to all enemies, with extra damage on Electric targets, concentrating damage in weakness phases.

In Electric boss fights, keep her Burst cycling so allies' Burst cooldowns stay uninterrupted, then check that S2 lands on Electric targets and holds 5 stacks.
In phases without Electric enemies, the damage taken increase and Electric extra damage do not apply, so swap the B2 slot to another DPS.

Raising Reload Speed with Resilience Cube shortens the time to meet the 30 normal attack condition. Ammo refill from Bastion Cube reduces gaps between frequent firing and Burst use.

Set Increased Elemental Advantage Dmg and ATK lines first, then use remaining lines on Critical Rate and Critical Damage to reduce variance in Burst and normal attack extra damage. Put Max Ammo only in leftover lines, just enough to reduce reload gaps.

S2's Electric damage taken increase is the core, so raise S2 first. Raise Burst above baseline for the extra damage window, and set S1 at the level that reliably triggers Burst cooldown reduction.

In campaign, it helps for the squad to have baseline Burst generation to benefit from the Burst Skill Cooldown reduction. In bosses and Solo/Union Raid, S2's damage taken increase and Burst extra damage are valuable in Electric boss fights. In Tower, the debuff only holds value in Electric phases, and in PvP her kit has no PvP-specific utility, so her priority is low.

When Full Burst Time starts, she reduces Burst Skill Cooldown for all allies, speeding up squad play.
S2's damage taken increase stacks up to 5 times on Electric targets, and her Burst also gains extra damage.
Even with the 30 normal attack hit condition, it relies on repeated fire, so she is simple to use.

S2's damage taken increase and her Burst's extra damage are tied to Electric targets, so their value drops otherwise.
S2 targets 1 random enemy, so stacking is slow if it misses Electric targets.
For her Burst cooldown reduction to translate into squad Burst rotation, you need a squad that can fill a 20-second Burst slot.