Get Loong Scales first from Chapter 2 after beating Flowing Sands boss need to break wall near First Rat Prince either make him slam it or use Wandering Wight Spirit Attack.

Head through crack for chest with scales, good to know you need all three other Loongs dead first Cyan Black and Red before trying Yellow.

Make your way through Chapter 4, beat Buddha's Right Hand then go to Middle of Webbed Hollow. Drop left past the rope bridge with Clawed Cocoons, Cloud Step helps here then up to shrine at Relief of Fallen Loong, Yellow waits in pavilion above.

Maybe interesting that Shock resistance helps tons here, grab some Shock-Quelling Powder or upgrade Gourd with Xu Dog before fighting. Immobilize spell works great to stop his big moves just watch your mana. Path to pavilion can be tricky to spot first time through, worth checking latest updates since requirements sometimes change with patches.