2 piece Gladiator's Finale , 2 Piece Viridescent Venerer

Edit: there is a new artifact set for him: Vermillion Hereafter

Best 5* weapon is Primordial Jade Winged-Spear.

Best 4* Weapon is Blackcliff Pole 

Also xiao is so sensitive to his ATK, try to make it as high as possible.