Details: After defeating the Elite Four and finishing the main storyline, the player can go to the Ruins of Hope at anytime and Battle the Poni Island Guardian Tapu Fini by interacting with the pedestal.
Tapu Fini will be at Level 60, knowing the Moves of Nature's Madness, Muddy Water, Aqua Ring and Hydro Pump. It is a Water/Fairy Type Pokemon. It has the ability of Misty Surge, which creates an Misty Terrain within the Battle. With this type of terrain, the power of Dragon Moves decrease by 50% if both Pokemon are on th ground. Pokemon on the ground will also not be affected by Status Conditions.
Since Tapu Fini is a stationary Legendary Pokemon, it can be battled again if a player were to run or accidentally defeat it. However, the player will have to defeat the Elite Four again before Tapu Fini reappears.