>>29495331Well, assuming its HA is Stakeout, then a clear counter emerges; Pursuit users. Particularly Bisharp, as it also resists Grass.
Say Decidueye comes in on a Water-type to kill it or force it out. Water-type switches out for Bisharp, who takes little from the incoming Spirit Shackle/Grass move. At this point it's boned, as it falls victim to the same Pursuit/Sucker Punch trap that plagues so many other Pokemon weak to Dark.
The only chance I see for Decidueye to do well, is if it gets some valid form of anti-Dark coverage.