Q&A for How to Catch Mareanie in Pokémon Sun and Moon

You have to proceed far enough into the story to get to Poni Island. From there, you have to enter the Steelix house and exit out the side door. There will be a fishing spot in which you can fish for Dhelmise. It might take you a while because of the low catch rate.

Make sure it doesn't know any fairy type moves. Also remember that Umbreon level up at night, and Espeon level up during the day.

Have your starter evolved all the way, then breed with Ditto. Trade that starter for the other starters. Or go on GTS and trade whatever the other person wants so you can get a starter.

Stock up on a bunch of potions and revives and use them when you have two Pokemon left and in between battles.

It is 100% random. The only thing a shiny charm does is increase odds. You can, however, do SOS hunting.

No. If you were to breed a shiny with a ditto or another shiny Pokemon, you will get another non-shiny version.

You need to get max friendship on the Pichu. I would recommend using a soothe bell.

They can only be found via SOS battles against Slowpokes in Kala'e Bay on Melemele Island.

Corsola can be found during the day or at night, but it has a very low encounter rate. Keep trying, and you should eventually find one.