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Make long reach have a Accuracy buff. Give Decidueye the move Grasswhistle. And he's basically broken. No one but other grass types would want to switch into it. So either lose a healthy water, rock, ground, ghost, psychic type to it or you swap and get out to sleep. And Decidueye will ofc set up.
I'm surprised Decidueye doesn't already learn this move. Maganium, sawsbuk and Sceptile(who also kinda has a beak) learns it too.