Xiao - Colossal Peng is a larger, more powerful Razorclaw Peng located on the Great Wall. He is part of the side quest Peng Problems, given by Zi Chan a little earlier. Defeating him updates the side quest, allowing you to get your reward from Bai Li Xi in the next region.
Xiao is basically a stronger version of a normal Razorclaw Peng. He uses the same long-range Sonic Wave attack, which causes skill interruption and fumble and can drastically reduce health. Occasionally he summons a few Gibbering Pengs. Being minions, they will die along with him, once he's defeated.
Xiao has basic elemental resistances (Fire, Cold, Lightning), and, like all bosses, is (almost) immune to Stun and Freeze attacks. However, he's susceptible to Poison and Vitality damage.
There's not much to say here. Just jack up your vitality resistance, and you may not even notice he is a boss.