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I've been a little apprehensive about sharing how to do quests and such because I typically run my mouth too much and spoil things, but this quest is very annoying and the first time I did it, it took me hours of searching to finish, you need to find the boss' Pokemon, there's 4 of them but only 3 actually matter for now, if you want save yourself a headache, look up the spoilers.
Hatterene is on route 2, Clefable is top of Silver Rise and copium bird aka Tokegiss is on top of Hardened Mountain.