Walkthrough: At the peak of Mount Lanakila, directly before challenging the Elite Four, Hau will show up from nowhere and challenge to a Battle of your own. Like Gladion at the base of the mountain, Hau and his team has improved in strides.
Trainers:
Hau (Rowlet Starter) - Level 53 Raichu, Level 53 Flareon, Level 53 Komala, Level 54 Primarina w/ Waterium Z
Hau (Litten Starter) - Level 53 Raichu, Level 53 Vaporeon, Level 53 Komala, Level 54 Decidueye w/ Grassium Z
Hau (Popplio Starter) - Level 53 Raichu, Level 53 Leafeon, Level 53 Komala, Level 54 Incineroar w/ Firium Z
Afterwards: After defeating Hau, he will give you three Max Revives. Hau will also promise that this will definitely not be the last Time the two of you face, that he will be all the more stronger next time around.