To venture deep into the caves of Volcanion in Saihao, and face off against the fiery deer Rio, you need to wear the water-spraying suit and pave the way to proceed.
Method to obtain the Saihao water-spraying suit: (If you already have it, please feel free to ignore this; thank you for your attention, dear players.)
It's very simple: head to the mechanical room, click on the water gun training in the lower left corner, and play a small game.