Let’s say you didn’t get Ash-Greninja and you need a Water and a Dark type. What are your options?
You only have one good option: and that’s to get a Sharpedo. Although Gyarados is available in Sun and Moon and it’s mega evolution is a Water/Dark type, you won’t be able to get its mega stone until after you beat the game. Crawdaunt, the only other Water/Dark type besides Greninja and Sharpedo, is not available in Sun and Moon. So where do you find one?
Unfortunately, it can only be found late in the game. What’s even worse is that they’re not easy to get. On Poni Breaker Coast, it can be found by fishing. If you’re fishing regularly, it only has about a 1% chance of appearing. If you fish in a bubbling spot, that chance increases to 10%. They can call for help, so if you want a Speed Boost Sharpedo, you can get one. Otherwise, it will have Rough Skin. The maximum level they can be found at is level 43 (and the minimum is 10, which makes it underleveled because Carvanha, its pre-evolution, evolves at level 30), so you might want to encounter a Sharpedo, have it call for help, and wait until a level 43 one (hopefully) appears. Alternatively, you can soft reset for one. At that level, it will know Crunch, Poison Fang, Scary Face, and Ice Fang. It may not know any Water type moves (and the only one it learns by level up is Aqua Jet, which it learns at level 11), but it knowing Ice Fang makes it better to use against Hapu’s Flygon. It’s not good to use against her Dugtrio, so it might be a good idea to teach it Scald (the TM for Scald can be found at the Brooklett Cemetery), Surf, or Waterfall (the TMs for both of these can be found at Poni Breaker Coast). I would suggest either Surf or Scald, though, because her Mudsdale has Stamina as its ability. It also knows Double Kick, so burning it would be good, but Scald only has a 30% chance of causing a burn. If your Sharpedo has a high special attack, then Surf might be a better move, so it has a higher base power. Her Gastrodon can be taken out easily with a Grass type.
Not only would Sharpedo be a good idea to use against Hapu, but it can also take out some of Olivia’s Pokemon. I would suggest using it against her Probopass and Lycanroc, because her Relicanth can be taken out easily with a Grass type, her Carbink knows Moonblast, which is super effective against Sharpedo (and can be taken out more easily with a Steel type move), and her Golem knows Thunder Punch, which is also super effective against Sharpedo (and can be taken out more easily with a Ground type move). Also, using Sharpedo against Acerola would be even better because it can take out more of her Pokemon. The only one you might want to think about using a different Pokemon is her Dhelmise because it knows Energy Ball, which is super effective against Sharpedo. In addition, Sharpedo could also be used against Kahili’s Oricorio, Kukui’s Incineroar (if you chose Rowlet) and Lycanroc.