Chenxing - Ethereal Cloud

If you don't have Chenxing - Ethereal Cloud yet but want to try her out, she is available as a Trial Operative in Story Mission SP1-12 (Chapter: Perilous Snowpath)

Being one of the early 5* operatives, Ethereal Cloud isn't very strong by current powercreep standards, but she is easy to play, is good against groups, has good range, has decent mobility, and can work with a lot of support operatives, which make her a good general-purpose operative.

The Basics

Role: Main DPS
Deals Electric Damage
Deals mostly ballistic damage with some skill dmg
Mobility is decent

How to use Chenxing - Ethereal Cloud

Use her standard skill to mark enemies
Use her standard skill whenever the marks expire
Spam her ult
Otherwise, just shoot stuff

How to build Ethereal Cloud

Weapon: Anti-Evil Ward / Wild Leer
Weapon Parts: Max ATK
Logistics Squad: Amano-Iwato / Mingyi (ADS/Hipfire)
+10.0% ATK
+10.6% Crit Damage Amplifier
+8.5% Electric Damage
Best supports:
Yao - Nightglow
Vidya - Celestial Swan

Specifics:

Standard Skill

Hold her standard skill to scan the area, marking up to 5 targets in front of you (or 8 targets with her Neuronic).
When release, Chenxing launches "Spiritual Slips" to all the marked targets, dealing Electrical Damage.
This can attach to enemies and parts, allowing the same enemy to have multiple Slips on them at once, as their parts can also have them.
Chenxing shooting a target with a Slip deals Electrical Damage to all enemies with Slips.

This skill lasts 10s on a 10s cd, so it can be maintained indefinitely.
It uses 20 S-Energy though, so eventually Chenxing will run out of S-Energy if she spams it.

Support Skill

Ethereal Cloud deals Electrical damage to up to 5 targets in front of you, and attaches a "Scarlet Thunder" to each of them.
Targets with a Scarlet Thunder also take Electrical DoT damage for 10s.
Shooting any target with a Scarlett Thunder deals damage to all targets with a Scarlet Thunder (with a 3s ICD)

The total damage of this skill is decent, but like all main-dps operatives, you would never want to use Ethereal Cloud as a support operative.

This skill has a 10s duration on a 20s cd, so it has 50% uptime by-default.
It also uses 20 S-Energy on a 20s cd, so Chenxing will by-default have no problems sustaining her S-Energy.

Ultimate Skill

Chenxing uses all Spirtual Slips and Scarlet Thunder to deal AoE Electrical damage in front of her.
This area is then bombarded for AoE Electrical Damage over the next 6s.

With its first Neuronic, the first hit also refreshes the duration of all Spiritual Slips on all targets it hits.
Since you can have 100S% uptime on them by-default this isn't overly useful, but since it doesn't use S-Energy it can give Chenxing some more time to recover S-Energy before needing to use it again, which can be useful for longer fights.

With its second Neuronic, it also increases the ballistic damage enemies in the AoE take by 20%.
Given that Chenxing deals almost entirely ballistic damage, this is a very strong buff.

This skill deals ok damage, but its high U-Energy cost means its dps isn't particularly high.

Weapon

Anti-Evil Ward
Ethereal Cloud's best weapon.
Increases her Electrical damage and Crit DMG Amplifier.
Every time Chenxing deals electrical damage, she gains a stack of Crimson Skyshield. At 50s tacks she gets +21/35% ATK for 5s.
Due to how Chenxings skills work, this has very high uptime, and you can generally maintain it indefinitely without too much issue.
Wild Leer
Increases ATK and gives even more ATK after shooting a weakspot.

For weapon parts Chenxing wants maximum ATK.

Logistics

Squads:

Amano-Iwato
Increases ADS Ballistic damage by 10% after a crit, up to +50%.
If shooting ADS, Amano-Iwato is Chenxing's best logistics set.
Mingyi
Increases ATK by 1% for 3s after dealing electrical damage, up to +45%. Repeated hits reset the duration.
If Chenxing is not ADS and is hipfiring, then Mingyi is her best logistics set.

Amano-Iwato gives a higher max boost, and can be kept at the max of 50% indefienitely provided Chenxing doesn't leave the field, making Amano-Iwato Chenxings highest-dps logistics squad if you're shooting ADS. If not shooting ADS then use Mingyi.

Stats:

Basic Stats: +10.0% ATK
Crit DMG Amplifier
Special Stat: +8.5% Electrical Damage

ATK and Electrical Damage for max damage.

Chenxing shoots weakspots a lot, and hits against weakspots always crit, so increasing her crit damage really helps Chenxing a lot.

Alignment Index does increase her shots damage too.
Ideally Chenxing is shooting weakspots a lot, and overally Crit Dmg Amplifier is the better stat, but Alignment Index does work well and works better if an enemy doesn't have any weakspots, although most bosses do have a weakspot and hence crit dmg is the better stat overall.

Manifestation

With M1, Ethereal Cloud shooting a weakspot causes them to take 1% extra Ballistic damage for 3s, up to 15 stacks.
Shooting the targets non-weakspots resets the duration of existing stacks, but doesn't add any stacks.
Since shooting things is what Chenxing does for most of combat, this is pretty easy to sustain, and its a useful if not great buff.
M2 increases the duration of all Chenxings Spiritual Slips and Scarlet Thunder by 50%.
I.e. M2 increases their duration from 10s to 15s.
Chenxing is a poor support operative, and this doesn't really make her any better.
Chenxing can sustain her Spiritual Slips at 100% uptime anyway as they have a 10s duration on a 10s cd, but M2 does help with sustaining her S-Energy.
Overall, M2 is a very weak buff.
M3 give Chenxing a +30% Crit Damage Amplifier buff when shooting a target with a Spiritual Slip.
This is a pretty nice damage buff.
M4 increases the level of her standard skill by 1, and increases the shot detonation damage to 28% (from 24%).
Its a nice damage buff, but M4 is quite costly.
With M5, whenever her standard skill detonates from being shot, there's a 30% chance to deal an extra 50% of the detonations damage.
Whilst hte chance is low, this does average out to quite a nice damage increase.

If you want efficiency (good damage without massive investment), then M3 is a good stopping point.
M1 is also a pretty good stopping point as M2 isn't overly useful for shorter fights.

Good Supports for Chenxing - Ethereal Cloud

Ethereal Cloud deals primarily ballistic damage, but can do a decent amount of skill damage.
As such she works well with hybrid supports or debuffers, as purely-ballistic and purely-skill supports won't buff all her damage:

Vidya - Celestial Swan for the resistance and DEF shredding
If using Celestial Swan, make sure the other support isn't off element to Chenxing.
Yao - Nightglow for the ballistic, ATK, and crit buffs. Nightglow is also on-element to Chenxing.
Enya - Exuvia works well due to Chenxing continuously dealing ballistic and skill damage. Exuvia is also on-element to Chenxing.
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.
Per-shot buffs like Tess - Phantasia work quite well
Fritia - Little Sunshine is the cope version, but Fritia works well for a 4* support.

For weaker cope supports you can consider operatives like Vidya - Agave for her ATK buff or Fritia - Little Sunshine for her ballistic damage buff.