The squirt bottle.
So there's this roadblock on the way to Ecruteak City on Route 36 where a Sudowoodo is blocking the path and you're told by a woman standing by it that it moves when splashed with water. I thought, logically, that this meant I needed a Water-type Pokémon to proceed. This seemed odd but I went looking anyway.
A frustrating hour later I resorted to the internet and the actual solution to this is pure adventure game moon logic.
There is a flower shop in Goldenrod City, which the lady on Route 36 says she works at. Once you have defeated the Goldenrod gym leader you can go to the flower shop and the NPC there will give you a squirt bottle that you can use to squirt water on the Sudowoodo which will make it attack you.
I'm sorry but that's obtuse and one of the most obscure solutions this series has ever thrown at me.
Fuck Johto 2k18.