Sacrificial Sword is by a large margin the best 4-star weapon on Xingqiu, who desperately needs the energy recharge and the energy particles from the extra elemental skill use. The 5-star Skyward Blade is maybe slightly better if you have it. Skyrider Sword is typically the best F2P option, but if your team can generate a lot of energy Harbinger of Dawn does significantly more damage. Festering Desire will be best F2P sword once 1.2 is released. If Xingqiu has less investment, Royal Longsword is also a decent option.
4 piece Noblesse Oblige is the best artifact set on Xingqiu. If you have a better Noblesse carrier on the team, you can run 2 piece Noblesse along with 2 piece Gladiator’s Finale, The Exile or Scholar sets.
You want around 180% energy recharge with Sacrificial Sword, and around 260% without. Using an Energy Recharge hourglass, Hydro Damage goblet and Crit Rate circlet is ideal unless you have a crit rate weapon, in which case you want a crit damage goblet. Using an attack circlet over crit rate is only a tiny DPS loss, so it’s worth it if your attack circlet has better substats. Using an attack goblet over hydro damage is a bigger DPS loss. Having Energy Recharge on the hourglass is necessary unless you have both Sac Sword and either good ER substats, Mona, or Childe.
Constellations massively increase Xingqiu’s power level, with C3 increasing his overall damage by 50% and C6 doubling it.
Xingqiu scales very well with investments into artifacts and weapons, but he’s also amazingly useful for setting up elemental reactions even if you invest almost nothing. A good cheap Xingqiu build would be level 20 or 40 Xingqiu with a level 40 energy recharge sword (Skyrider Sword, Sacrificial Sword or Favonius Sword). For artifacts, use 2 piece Scholar 2 piece Exile, all +0, with +5% energy recharge substats where possible.
Although Xingqiu has some healing from his passive talent, it’s not enough to make him a real healer, even with Maiden’s Beloved and a healing bonus circlet. It’s still really helpful in keeping your team alive, though.
Elemental Mastery does almost nothing on Xingqiu, because his burst applies Hydro so fast he will be the character applying Hydro status to the enemy for another character to react with.