The final tapu’s ruins are the easiest to reach and travel through, on account that you had to physically reach the altar to progress the story. Anyway, as a reminder, you’ll find the Ruins of Hope near the south side of Poni Island. Unless you’re walking there or something, call up Charizard and have it fly you to the glide spot.
After entering the ruins, simply head north while keeping towards the east side. The block that you pushed into the gap is still there, making it super easy to reach the doorway at the end. Upon making it to the altar, walk up to where Hapu got her sparkling stone and, as usual, save your game before challenging the tapu.
The mystical Tapu Fini has excellent Special Defense and decent Defense, making it the most defensive of the four tapu. At the start of battle, its Misty Surge Ability will cover the landscape with Misty Terrain, which weakens Dragon-type moves and confers immunity status conditions for Pokémon on the ground.
It can use Hydro Pump and Muddy Water for STAB–the former is quite powerful, while the latter may lower the opponent’s Accuracy. It also has Aqua Ring, which slowly restores its HP each turn, and Nature’s Madness for halving the opponent’s HP. Attack-wise, it’s not too bad, but it can be a nuisance with Aqua Ring.
When choosing a type, anything that resists Water is good, such as Water itself, Grass or Dragon. Obviously, Grass-types should be careful when using their STAB moves. Also, Dragons can’t use their Dragon-type moves seeing as Fini is a Fairy, and should instead use their secondary STAB moves or non-STAB moves.
Of the four tapu, Fini is probably the hardest one to catch. Firstly, Misty Terrain prevents Tapu Fini from being affected by status conditions for five turns. If you’re impatient, you can overwrite Misty Terrain with your own terrain, via another tapu for example. Secondly, Fini is able to slowly regain HP using Aqua Ring.
Unlike how Tapu Bulu regained health with Grassy Terrain, Aqua Ring never runs out, so if you fight Tapu Fini normally, you’ll have to keep chipping away at it every now and then. Which is highly inefficient, since you want to use those turns to throw Poké Balls instead. Now there are a couple of ways to deal with Aqua Ring.
The simplest method is to inflict Tapu Fini with a burn using Will-O-Wisp. A TM for this move can be obtained by traveling west from Diglett’s Tunnel in Akala Island. If you need help, refer to our Machamp Detour page. Naturally there are two things to consider.
One, you need to wait until Misty Terrain is gone or remove it yourself. Two, Will-O-Wisp is generally used by Fire-type Pokémon, which are weak to Tapu Fini. There are some non-Fire Pokémon that can learn it though–most notably, a bunch of Ghost-types including Gastly, Drifloon and Misdreavus, plus their evolved forms.
As soon as Misty Terrain is gone, use Will-O-Wisp for a 85% chance to cause a Burn. The damage from Burn will cancel out the healing from Aqua Ring exactly. On top of that, it makes it easier to catch Tapu Fini, as if you inflicted paralysis. To further improve your odds, you can use Net Balls from the Paniola Town Poké Mart.