Chenxing - Jade Arc (a.k.a. "JA")

If you don't have Chenxing - Jade Arc yet but want to try her out, she is available as a Trial Operative in Story Mission 14B-9

Jade Arc has a short and simple rotation that deals extremely high damage, with a 100% guaranteed crit rate for all of her standard damage rotation, never needs to reload if played optimally, can be supported by a lot of strong support operatives, and she can also potentially stack enough arrows to deal enough damage to bypass invuln phases.
JA's main downsides are her low bulk and mobility that can also make her squishy, but these are small downsides and overall JA is an extremely strong operative.

The Basics

Role: Main DPS
Electric Damage
Deals mostly skill damage
All of her damage benefits from ballistic buffs.
Mobility is poor

How to use Chenxing - Jade Arc

Use her Standard Skill
Shoot three Stormwake Arrows
(Standard shots that automatically release)
If her ult isn't off cooldown, fire a regular arrow
(This isn't required for the rotation, it just helps)
Ult when she has 4 [or more] embedded arrows

Repeat

How to build Chenxing - Jade Arc

Weapon: Skybreaker / Hidden Hero
Weapon Parts: Max ATK
Logistics Squad: Couplet
+10.0% ATK
+10.6% Crit Damage Amplifier
+8.5% Electric Damage
Best supports:
Yao - Nightglow
Vidya - Celestial Swan

Specifics:

Standard Attack

Hold the trigger to have Jade Arc draws her bow back.
You can then release the trigger to shoot the arrow.
If her bow is fully drawn back, the arrow deals more damage, as well as having a 100% Crit Chance.
You should thus always fully draw her bow back.
Unless parts breaking - if trying to Parts Break with Jade Arc you should spam standard arrows as quickly as you can - don't fully charge them.
You're almost certainly better off swapping to another operative if you need Parts Break though... [e.g. Agave]
Each fully charged arrow that hits will embed 1 arrow into the target.
These embedded arrows are where most of Jade Arc's damage comes from, so you should never fire arrows that aren't fully drawn back [again - unless Parts Breaking].

Standard Skill

Jade Arc fires a vast number of [72] arrows at enemies in front of her.
All of these arrows always crit, and deal +100% damage to shields much like actual ballistic damage.
These arrows seek out targets automatically and auto-aim - they can't miss.
The auto-seek range is lower than the projectiles range, so you can fire at a target that is out of range of the auto-seek and the skill can still potentially hit them.
They are spread out roughly evenly against possible targets. If there are fewer targets each will receive more arrows. If there is one target then all the arrows will hit them.
Arrows deal less damage against a target that has already been hit by an arrow.
This means that the more enemies the skill hits, the less the damage is reduced, and hence the higher its dps, but it's still decent against single targets.
These arrows also count as hipfire ballistic damage in addition to skill damage.

If a target is hit by this skill, the first arrow is embedded into them.

Jade Arc can move during her Standard Skill, but she moves very slowly.
Jade Arc can't be interrupted whilst casting this skill.

Each target hit by this skill reduces its cooldown.
This makes it very spammable, and with enough targets you can use the skill again before it's finished, allowing Jade Arc to continually fire it.
This is very useful in certain game modes, such as Paradoxical Labyrinth - Battlefield Sigma, where you can fire it continuously for a staggering RoF.

This skill also charges three StormWake Arrows.
This skill on its own isn't that strong against single targets - it's primarily used to build charged arrows and embed arrows into the target.

Stormwake Arrows

These are special arrows that fire automatically when fully charged (they don't need to be manually released), deal more damage, and automatically home in on targets.
They also give less U-Energy recovery and deal less destructible object damage.
This is notable - If trying to parts break don't use Stormwake arrows.
Again though - for Parts Breaking Jade Arc should spam uncharged shots (or ideally swap to an actual Parts Breaker like Agave)

Ultimate Skill

Jade Arc extracts all the embedded arrows in every target, dealing absolutely massive damage.
Jade Arc's Ultimate is where most of her damage comes from.
Jade Arc's ultimate is guaranteed to crit, and deals +100% damage to shields (much like actual ballistic damage).

Jade Arc's ultimate deals more damage the more arrows are embedded (this is linear), with extra damage for the first 4 embedded arrows.
Her standard skill embeds 1, and each of her 3 Stormwake Arrows also embed 1 each, so you can use her standard skill, fire 3 Stormwake Arrows, then Ultimate with 4 embedded arrows.

Note that her ult deals regular damage even if the embedding arrow hit a piece of enemy armour.
This can make it easy to defeat armoured enemies, as the damage from your Ultimate can deal damage to enemies directly, bypassing their armour pieces.
Against some enemies you can basically ignore their armour entirely.

Jade Arc's Ultimate doesn't use any U-Energy.
This allows you to have all of your U-Energy be used for a support operative instead.

For each arrow extracted, Jade Arc recovers 1 arrow back to her "magazine".
This means she potentially never needs to reload.
Note her Standard Skill doesn't use any ammo, but can embed arrows, allow her to refill her "magazine" through her standard damage rotation.
This means that even if you mess up a rotation you will likely still never need to reload.
Conversely though, against weak enemies who die before Jade Arc can ult, she may not be able to recover any arrows at all.
Against groups of weak enemies you are generally better spamming your standard skill though rather than shooting Stormwake Arrows.

Other Notes:

Note that you can still embed arrows into invulnerable enemies.
You can thus continue shooting enemies during invuln phases to deal a massive burst of damage with your Ultimate when their invulnerability ends.

Alternatively you can embed arrows into enemies until they are just before their invulnerability trigger point, and then Ultimate.
This can potentially allow you to deal enough damage to kill the enemy outright, thus skipping the invulnerability phase completely (This doesn't work on all enemies).

If you are up against a large group of very strong enemies, you can spam her standard skill 4 times - this will embed 4 total arrows into each enemy.
You can then Ultimate to deal max damage against all enemies. This is more efficient at doing group damage.
With a well-built Jade Arc, you are almost never going to encounter a group of enemies this strong.
The only place you are likely to want to do this is in Paradoxical Labyrinth - Battlefield Sigma or other event-specific challenge modes, but it is useful in such modes.

Support Skill

Jade Arc creates an AoE DoT areas that deal Electrical damage, which also inflicts Paralyze for 4s.

Like all Main-DPS-ers,. Jade Arc's Support Skill is not very good and you really should never be in a situation where you want to use it.

Weapon

There are essentially only three bows in the game - the blue 3* one she comes with by-default (Combat Rite) , the 4* event weapon (Hidden Hero), and her 5* (Skybreaker).
There is technically the standard 2* one as well (Plum Branch) but since Chenxing comes with the 3* which performs the same, there's never a reason to consider the 2*.
Obviously her 5* is best, and her 4* is better than the 3*, and the 3* is better than the 2*. Simple.

For weapon parts - you have a Hobson's choice as there's only one option for each slot - equip them, or don't.
So equip them if you have them.

Logistics

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

For stats you want 10.0% ATK and 10.6% Crit Damage.
For the Special talent, you want 8.5% Electrical Damage

Both opf Jade Arc's skills always crit, and her skills are where most of her damage is. Similarly, fully charged shots also always crit. As such Crit Rate is not very useful (literally useless for her standard rotation), and crit damage is very useful.
Note that JA has a 100% base Crit DMG stat, meaning a Crit DMG Amplifier stat does add a lot to her damage (+10.6% Crit Dmg Amplifier = +10.6% Crit DMG)

Alignment Index does give Crit Damage Amplifier, but 106 Alignment Index only gives 8.48% crit damage amplifier, so you're better off getting the 10.6% Crit Damage Amplifier roll instead. A high-roll Alignment Index mod is still respectable and a decent cope stat though.
All other stats are useless to Jade Arc.

Manifestation

M1 gives Jade Arc a 30% Final DMG boost for her ult if the target has at least 4 embedded arrows.
Given that the target should always have at least 4 arrows because Jade Arc's standard damage rotation embeds 4 arrows, this should ways be applying to JA's ult.
Similarly, since JA's ult is where a large part of her damage comes from, the overall damage increase to her rotation is high.
With M2, each shot from her Standard Skill inflicts a -1% DEF debuff for 5s, up to 20% max.
This means that you'll want to use JAs ult within 5 seconds of her Standard Skill. This is not difficult and you'll normally do this anyway.
As such, this is basically just a 20% DEF debuff, which is good.
M3 gives JA 5% ResPen per arrow embedded into the target, up to 15%.
This isn't quite a 15% ResPen boost for her entire damage rotation, but given how much of her damage is back-ended into her ult it is actually very close.
M4 gives JA's ult +1 level, as well as increasing the damage per arrow to 143% ATK + 312.
Since the base is 130%, this translates to around 10% effective/final damage increase for the damage part.
M5 gives +15% ATK, and an additional +10% ATK for 10s after attacking a target inflicted with a control effect.
Note that JAs 4* and 5* bows do inflict a debuff on the enemy, which have essentially 100% uptime, but these aren't control effects.

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

Good Supports for Chenxing - Jade Arc

Jade Arc's entire damage rotation counts as ballistic damage, and her skills also count as skill damage.

Since her Stormwake Arrows don't count as skills, JA is better off with ballistic buffs rather than skill buffs, although skill buffs still work very well with her.

Working well with ballistic and skill buffs mean that most support operatives work with JA to at least some extent though.
A list of good supports for JA is basically just a list of most of the good supports:

Vidya - Celestial Swan for the resistance and DEF shredding
If using Celestial Swan, make sure the other support isn't off element to the main DPS.
Yao - Nightglow for the ballistic, ATK, and crit buffs
Siris - Freefin for the skill damage buffs, although JA can't make use of Freefin's Wavehunt thermal skill damage buff.
Acacia - Kaguya for resistance debuffing if Vidya is busy that week
Eatchel - The Cub's ATK boost is still very nice.
Mauxir - Shadow Ka still works well for the extra damage and having an extra target for her Standard Skill to hit.
Enya - Exuvia works quite well due to her shooting and skill use, but isn't ideal due to JAs low RoF and skill use rate

For weaker cope supports you can consider operatives like Vidya - Agave for her ATK buff or Chenxing - The Observer for her Skill DMG buff.