Acacia - Chrono Echo

If you don't have Acacia - Choron Echo yet but want to try her out, she is available as a Trial Operative in the Events tab.

Chrono Echo offers the ability to swap between massive AoE damage and strong single-target damage basically at-will, with her Flying Daggers having the same optimal range as an SMG and about the same max range as an Assault Rifle, which gives her great range and thus pretty good survivability too.
The main downside of Chrono Echo is that she's much more complex than other operatives, with a more complex rotation and more things to keep track of in order to play her optimally. Once mastered though she's extremely capable, and works well for all content.

Overview

Role: Main DPS
Deals Kinetic Damage
Weapon Type: Flying Sword
Deals skill damage
Mobility is good

How to use Acacia - Chrono Echo

Use her standard attack until she has 16 Phase Energy
(The counter below the Moon in the centre of the screen)
Use her ult to swap to her Awakened/MILF/ult form

and then:

Against Groups:

Spam her standard skill to slash in front of you
(You can stay in her Awakened/MILF/ult form indefinitely)

Against Single-Targets

1. Keep attacking using her regular attack
  If this hits the same target 3 times, then:
  Hold her Standard Skill to slash in front of her
2. If you've done the Standard Skill slash from Step.1 twice, then Hold her Standard Skill to do an even bigger AoE slash
3. Repeat steps 1 and 2 until at 36 Phase Energy
  When at 36 Phase Energy, immediately tap her ult to do an AoE
  (It deals more damage the fewer targets it hits)

How to build Acacia - Chrono Echo

Weapon: Moonwake Blade / Iron Edict
Weapon Parts: Max ATK
Logistics Squad: Sacred Tree
+10.0% ATK
+106 Alignment Index
+8.5% Kinetic Damage
Best teammates:
Vidya - Celestial Swan
Siris - Freefin
Marian - Riptide
Eatchel - The Cub

Specifics:

Attack Range and other Notes

Whilst Acacia's "Flying Sword" weapon might sound like it has a low/melee range, it actually has an 80m max range, with max damage within 30m.
30m is the max range of a shotgun. Shotguns deal max damage within 15m.
30m is also the max range that SMGs deal max damage in (with a max range of 60m).
Assault Rifles have a max range of 85m (and deal max damage within 50m).
As such, Acacia's Flying Daggers do actually have a very long range - they out-range SMGs, and have about the same max range as Assault Rifles.
They out-range all weapon types except Sniper Rifles, Assault Rifles, Semi-Auto Revolvers, Bows, Crossbows, and Gatling Guns.
Chrono Echo thus out-ranges most other operatives in the game.

Each hit Acacia does grants her1 Phase Energy, up to a max of 16. Phase Energy is used for her ultimate.
Some specific parts of her kit grant her more per hit, but unless otherwise stated in the below, each hit gives her 1 Phase Energy.

All of Acacia's projectiles have auto-aim, and fire towards nearby enemies, even if you aren't directly looking at them.
This isn't as generous as some other operatives - you do need to be aiming pretty near to enemies to get Acacia's auto-aim to work.

Basic Attack and Standard Skill

Tap Acacia's Standard Skill to swap her between Orbital and Converged forms.
In the centre of the screen is a blue Crescent Moon icon. You can tell which mode she's in from the icon in the middle of that Moon
Orange Crescent Moon = "Orbital Moonedges" i.e. "AoE mode" [This is the default mode]
Red Hooded figure = "Converged Moonform" i.e. "Single-Target mode"

Acacia's regular attack will throw four daggers at the target, dealing Kinetic-damage.
Orbital Moonedge Whilst in Orbital Moonedge form, each dagger hit also deals the same amount of Kinetic-damage to up to 4 nearby enemies.
Converged Moonform Whilst in Converged Moonform form, each dagger hit also deals damage to the target four extra times.
Whilst Orbital Moonedge's extra hits deal the same damage as the initial dagger, Converged's extra hits deal a bit less damage than the dagger.

Each of these 4 daggers deplete 1 "bullet" from her magazine, which has a base capacity of 16, so she can by-default attack 4 times before having to reload.

Ultimate Skill

If Acacia has 16 Phase Energy, you can tap Acacia's ultimate skill to Awaken her. Her physical/visual form changes as well:
When not-Awakened, Acacia uses her [usual] teen form. Teen-form Acacia takes 50% reduced DMG
When Awakened, Acacia uses her older/MILF form. MILF-form Acacia deals 50% more Final DMG

Note that Acacia's Awakened state [and Phase Energy] persists even when she's not on-field, and whether she is in her Awakened form is [somewhat] relevant for her Support Skill.
Note also that Acacia's portrait in the top-right does not change when she's in Awakened form, so there is no way to tell if she's in Awakened form or not when she isn't on-field.
If she's in Awakened form, you can't use her ult whilst she's off-field, but you also can't use it if she doesn't have any Phase Energy, so her not being able to ult isn't an indicator.
It is possible this is a bug, but I suspect this is merely an oversight on the devs part - they didn't think to implement such an indicator, despite it being mechanically useful.
If you happen to see or know a Snowbreak dev, please jab them with a pointy stick until they change this.
This is a very minor issue since her support skill is mostly useless, but it is an issue.

Whilst in Awakened form:

Her max Phase Energy is increased from 16 to 36
Her normal [dagger] attacks are replaced by a single-target dagger projectile.
Each hit also inflicts it with 1 "Time Scar", used for the slash below.
Note that whilst in Awakened form, Acacia doesn't have a magazine and doesn't need to reload - she can spam this infinitely.
Each third hit deals extra damage. (Around 50% more)
Using her Standard Skill will deal a massive arcing slash in front of her, which is great against groups.
If this hits a target with max Time Scar, it deals double damage and removes all Time Scar, and Acacia gains 2 Phase Energy, and 3 "Radiant Tide".
Radiant Tide are the diamonds around the outside of her moon indicator.
This has a cooldown of 3 seconds, but with its second Neuronic the cooldown is reduced by 1 second per target hit, so against groups you can just keep spamming it.

When Acacia has 6 Radiant Tide, you can hold her Standard Skill to consume all Radiant Tide to do an AoE blast that deals high Kinetic-damage to up to 10 targets in front of her, which also grants her 6 Phase Energy if it hits.
Granting 6 Phase Energy allows Acacia to get back into her ult quicker in her next rotation.
With its first Neuronic, this also gains +15% Final DMG for each target below 10 that it hits, making it very good against single-targets.

Whilst in Awakened form, you can tap her ult again to deal Kinetic-damage to the target, and also deal 50% of that damage to up to 4 nearby enemies.
This skill has no base damage - its damage is entirely determined by how much Phase Energy Acacia consumes for it.
Using this removes her Awakened form and removes all Time Scars, but with its Neuronic it does refund her some Phase Energy based upon how much she uses.
This makes it much easier to get into her ult form for her next rotation.

Support Skill

Acacia creates a AoE that deals Kinetic damage.
This AoE starts in front of Acacia and slowly moves away from her.

If she's in her Awakened form, the AoE deals damage more often.

Whilst not stated in its description, this AoE lasts for around 6 seconds.

This AoE can be inconsistent against small enemies, and enemies can potentially move out of the way of the AoE before it gets to them, meaning it's not overly reliable for general gameplay, but against larger bosses it can perform decently well.
...or it can perform horribly - It really can be all over the place depending how many hits actually connect and where/if the enemy moves before it gets to them.

Its 15s cooldown means its dps isn't necessarily that good, and like all main-dps operatives you aren't going to want to use Acacia as a support.

Weapon

Moonwake Blade
Her signature 5*, and her best weapon.
Gives ATK and Kinetic DMG, as well as buffing her ult.
After using her ult, dealing kinetic damage causes enemies to take increased skill dmg.
As usual for 5* weapons, enemies taking increased damage is very strong, and is the main reason Acacia wants this weapon.
Iron Edict
Gives Kinetic damage, as well as Skill DMG for 15s after using her ult.

Since Acacia is the only Flying Sword operative in the game, these are the only weapons you can upgrade her to.
If you don't have either of these, then you'll have no choice but to use the default weapon she comes with.

For weapon Parts Acacia wants max ATK.

Logistics

Squad

Use Sacred Tree squad - it was specifically designed for her and is her best choice.

Sacred Tree gives her Kinetic damage when not in her ult form, and when in her ult form, gaining Phase Energy (i.e. doing anything) increases her standard skill damage for 2s.
It also increases the damage of her ult by 100%, which is fantastic.

Stats

Acacia wants:
Basic Stats: +10.0% ATK
+106 Alignment Index
+8.5% Kinetic Damage

ATK and Kinetic damage to buff her damage
Alignment Index gives her more ATK
A max-roll of +106 Alignment Index gives Acacia +8.48% ATK, wihch is very good.

Acacia can't heal or create shields.
She doesn't use U-Energy as her ult has a U-Energy cost of 0.
U-Energy Regen can be useful to help fuel a support operative, so whilst it is very weak, it can technically benefit her, and is thus an extremely weak cope stat.
Her standard skill has a U-Energy cost of 3, but you only need to use it like once per fight anyway, so she doesn't ned S-Energy Regen.
She deals skill damage, and skills can't crit, so crit mods are useless to her.
(and even if she could crit, her crit dmg is 0%, so crit mods wouldn't affect her damage anyway)
She also has no cooldown issues, so Skill Haste is useless to her.

Manifestation

M1 allows Acacia to ignore some enemy DEF after the slash from her ult removes some Time Scar.
This is something that should normally happen when fighting bosses anyway.
This is removed after using her ult blast, but you'll stack them again before her next ult.
You won't max this at 20% DEFignore if doing her normal rotation, but even 14% DEFignore is pretty good.
With M2, regular hits from her ult increase the skill damage enemies take for 3s, which is a very good damage buff.
This requires a Kinetic-weapon equipped, however since every Flying Sword in the game is Kinetic-damage, you can't not have this active.
With M3, having any of the DEFignore from her M1 increases the Final DMG of her ult blast by 30%.
Again - you'll be stacking that anyway, so the damage increase from M3 should be a basically permanent effect.
M4 give the usual +1 level to her ult, increasing its overall damage by 10%.
A nice damage buff, but that's all.
M5 gives Acacia +12% ATK, which is nice.
This requires a Kinetic-weapon equipped, however since every Flying Sword in the game is Kinetic-damage, you can't not have this active.

If you want efficiency (good damage without massive investment), then both M3 is a good stopping points.

Good teammates for Acacia - Chrono Echo

Acacia deals skill damage based on her ATK, and wants to debuff enemy DEF and resistance.

Good supports for Acacia are thus:

Vidya - Celestial Swan
DEF and Resistance shredding are excellent for Acacia
If using Celestial Swan, make sure the other support isn't off-element to Acacia
Siris - Freefin
Freefin provides strong skill damage buffs, although Acacia can't make use of Freefin's Wavehunt thermal skill damage buff.
Marian - Riptide
Riptide provides some nice skill damage buffs.
Since Acacia doesn't lose her Awakened status or Phase Energy when swapping off-field, Riptide can swap in whenever she needs to.
Riptide is off-element to Acacia (Riptide deals Chaos damage)
Eatchel - The Cub
A nice passive ATK buff.
Acacia - Kaguya is good for resistance debuffing if Celestial Swan is busy that week.
Chenxing - The Observer
Chenxing's Skill DMG buffs are good for Acacia, although Chenxing is still only a 4* and thus isn't that strong.