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.
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.
- Death Ward: to block the sonic wave's unique HP-reduction type of damage (does not seem effected by vitality resistance. (On legendary, his sonic wave will kill you in 2 tics even with 80 vitality resistance)
There's not much to say here. Just jack up your vitality resistance, and you may not even notice he is a boss.