Reaching the ruins of Tapu Lele is slightly faster–there’s a Charizard glide spot that takes you straight to the entrance. In case you’ve forgotten, the Ruins of Life is located towards the south side of Akala Island; it’s where you went to battle Olivia during her Grand Trial. Once you’ve landed, head straight into the interior of the ruins.
Inside, there’s a Machamp Shove puzzle similar to the one in the Ruins of Hope, which you had to solve during the main story. First, approach the giant block along the east side, then shove it into the hole just above it. Standing on top of the sunken block, shove the nearby block to the left so that it falls into the gap to its west.
While atop of this block, push the block nearby above you until it falls into the gap up ahead. Now you can proceed to the altar. Within, the layout is exactly the same as the other tapu’s ruins. After climbing up the stairs along the sides, save your game just in case. Then touch the tapu’s statue to initiate the battle.
This pink-shelled tapu boasts exceptional Special Attack and reasonable special bulk, but poor physical bulk. As the battle begins, its Psychic Surge Ability will trigger Psychic Terrain, which is a field effect introduced in Sun and Moon that boosts the power of Psychic-type moves and disables priority-moves.
Attack-wise, Tapu Lele can use Extrasensory and Moonblast for STAB–the former has a chance of causing flinch. It can also use Nature’s Madness like all the other tapu, which halves your Pokémon’s current HP, and Flatter to boost your Pokémon’s Special Attack and cause confusion, similar to Swagger but far less dangerous.
Because of Tapu Lele’s typing, Steel-types are perfect to resist its STAB moves. Of course, you need to exercise caution as the tapu is weak to Steel-type moves and not great at taking physical hits. Also, Psychic Terrain-boosted Extrasensory can still hurt, so don’t hesitate to up your Steel-type’s Special Defense with items etc.
Fortunately, Tapu Lele isn’t immune to any status condition, so you’re free to use Thunder Wave or other moves to paralyze it, or if you prefer, you can send it to sleep. The only other thing to keep in mind is priority moves like Quick Attack being negated by Psychic Terrain, but you shouldn’t need to rely on those during this fight.