Xiao is an Anemo polearm damage dealer that is both swift and hard hitting. His damage revolves around his plunging attack, which already has higher multiplier than most characters, and is further boosted by his elemental burst at the cost of some HP drain. Being a polearm character, he attacks pretty fast and the elemental skill makes him quite mobile and can be used multiple times for some burst damage. Outside of his damage dealing ability, he does not provide much as a support/utility, other than his skill/passive for traveling.
As a standalone damage dealer, the usual DPS build of 2 piece elemental artifact with 2 piece Gladiators Finale will work just fine. However, using a 4 piece Viridescent Venerer does have good potential to combo with your team. The elemental resistance shred that it gives will make your team deal more damage, and possibly make it worth the lack of 18% attack from using gladiators 2 piece. Since his character ascension bonus gives him crit rate, it will make building for crit damage more appealing, and just get crit rates from the substats.
For the weapons, Primordial Jade Winged-Spear may potentially be the weapon of choice for plunge attack spam playstyle. Skyward Spine can be promising if you end up struggling with his elemental burst uptime. For more obtainable weapons, Blackcliff Pole will be good on him under certain conditions, while Deathmatch is a more consistent option. Of the 2 craftable weapons, Prototype Starglitter helps with energy recharge to reach his burst faster, while Crescent Pike is good if your preferred playstyle is normal attacking, but the secondary stat of phys damage increase will be wasted since his burst imbues his attacks to Anemo element.
Using 4 piece Viridescent set bonus can be pretty strong if you pair him up with characters that can deal damage as you are attacking, like Xingqiu, Beidou, or Kaeya. His fast attack speed can also make really good use out of a c6 Fischl, increasing the damage of Oz's every shot because of viridescent bonus.
Another Anemo character can be a nice bonus for the elemental resonance. The Anemo main character can reduce an enemy's Anemo RES through their free constellations to help Xiao. Jean is another option because she can fill in the role of a healer too. If you have Jean at constellation 2, then she can also increase Xiao's attack speed.
Venti is a very good energy battery as well. Short elemental skill cooldown, and 4th ascension passive gives 15 energy too. Sacrificial Bow for even more energy!
Zhongli's elemental skill can reduce all enemy's elemental RES, as well as providing your team with a shield, making it safer to play aggresively when you use his elemental burst.
Chongyun's elemental skill increases melee character's attack speed, and Xiao's elemental burst prevents his attacks from being converted to Cryo element so Anemo goblet mainstat won't be wasted. Constellation 2 Chongyun reduces cooldown time of skills as well
Albedo's constellation level 4 increases plunging attack damage of character's within his elemental skill by 30%.
Tartaglia (Childe) has a passive that increases normal attack talent by 1, which will increase Xiao's plunging attack damage as well.
Xiao dons the Yaksha Mask that set gods and demons trembling millennia ago.
Greatly increases Xiao's jumping ability.
Increases his attack AoE and attack DMG.
Converts attack DMG into Anemo DMG, which cannot be overridden by any other elemental infusion.
In this state, Xiao will continuously lose HP.
The effects of this skill end when Xiao leaves the field.
Transcension: Gravity Defier
Decreases climbing Stamina consumption for your own party members by 20%. Not stackable with Passive Talents that provide the exact same effects.
Conqueror of Evil: Tamer of Demons
While under the effects of Bane of All Evil, all DMG dealt by Xiao increases by 5%. DMG increases by a further 5% for every 3s the ability persists. The maximum DMG Bonus is 25%.
Dissolution Eon: Heaven Fall
Using Lemniscatic Wind Cycling increases the DMG of subsequent uses of Lemniscatic Wind Cycling by 15%. This effect lasts for 7s, and has a maximum of 3 stacks. Gaining a new stack refreshes the effect's duration.
Increases Lemniscatic Wind Cycling's charges by 1.
Annihilation Eon: Blossom of Kaleidos
When in the party and not on the field, Xiao's Energy Recharge is increased by 25%.
Conqueror of Evil: Wrath Deity
Increases the Level of Lemniscatic Wind Cycling by 3. Maximum upgrade level is 15.
Transcension: Extinction of Suffering
When Xiao's HP falls below 50%, he gains a 100% DEF Bonus.
Evolution Eon: Origin of Ignorance
Increases the Level of Bane of All Evil by 3. Maximum upgrade level is 15.
Conqueror of Evil: Guardian Yaksha
While under the effects of Bane of All Evil, hitting at least 2 opponents with Xiao's Plunging Attack will immediately grant him 1 charge of Lemniscatic Wind Cycling, and for the next 1s, he may use Lemniscatic Wind Cycling while ignoring its CD.
A Stella Fortuna is required to unlock each constellation. See Constellations Guide for more details.