Xiao build recommendations

Regarding Xiao's Optimal Skill Rotation
Skill -> Skill -> Burst -> Plunge Combo

Regarding Xiao's Plunge Combo

Best combo for Xiao when fighting a single target
Plunge -> Normal Attack -> Charged Attack -> Plunge

Best combo for Xiao when fighting multiple enemies
Plunge -> Normal Attack -> Plunge

Xiao has issues with Energy regeneration because he cannot generate any particles with his Elemental Skill while his Burst is active, so it is not suggested to use his Elemental Skill during Elemental Burst.

Start off a fight by casting the skill of your Anemo Battery and then casting his skill twice then cast your burst immediately. You will want to be half way through your burst animation before the particles reach you, so this way you can start off your burst with a portion of the energy gauge already filled.

You'll want to pair Xiao with another Anemo character (Venti, Sucrose, Jean or Anemo Traveler) to help him gain energy while his Elemental Burst is on cooldown. Xiao's Elemental Burst ends when you swap to another character. Although he has two charges of his Elemental Skill, only ONE charge comes back per cooldown.

Regarding Weapon Choices

With a proper anemo battery (e.g. Sucrose), Xiao needs as little as 120-130% ER to maintain a high burst uptime. In these scenarios, unless there are no ER from substats, you should always go for a non-ER weapon as it will result in more damage.

Calamity Queller
This weapon has a really high Base ATK, which means you benefit more from ATK% buffs (e.g. Thrilling Tales of the Dragon Slayers) if compared with other weapons.

Regarding Artifact Sets

Desert Pavillion Chronicle (4)
If Xiao has a weapon with low base attack in a team with a lot of external ATK buffs like Bennett or Thrilling Tales of the Dragons Slayers this artifact set can be equal to or better than Vermillion Hereafter (4)

Prioritize Viridescent Venerer (2) over the other 2 +18% ATK sets if there are external ATK% buff sources on your team.

Regarding Artifact Main Stats

Anemo DMG% goblets are recommended for Xiao. ATK% goblets serve as a good alternative for him.