>>52430855There's a beach south of the electric gym that has a river in the middle that works well.
With a Fairy sandwich Marill is the only thing that spawns on the beach making it very easy to see. About 10 Marill spawn in on the beach and if you don't see a green one you jump across the river to the other beach to spawn them there. Just jump between the two beaches and you'll be seeing enough Marills that it will show up eventually.
Make sure to turn off autosaves, save before making the sandwich and only save once you see the shiny Marill.
