Walkthrough: Immediately after defeating the mysterious Ultra Beast at Aether Paradise and then being taking to Ula'ula Island by Wicke, Hau will challenge you to a Battle before you can even officially enter Malie City. His team has gotten exponentially better.
Trainers:
Hau (Rowlet Starter) - Level 28 Raichu, Level 28 Flareon, Level 29 Brionne (Z-Crystal)
Hau (Litten Starter) - Level 28 Raichu, Level 28 Vaporeon, Level 29 Dartix (Z-Crystal)
Hau (Popplio Starter) - Level 28 Raichu, Level 28 Leafeon, Level 29 Torracat (Z-Crystal)