No one has suggested it? OK, well... Octillery. It doesn't move much, hunts by patiently waiting for prey while suckered onto rocks and mainly attacks using the Beam moves, which imply accuracy at extreme range. I imagine it also camouflages itself (being an octopus), much as snipers do, and to top it all off it has the ability Sniper, indicating that it has a knack for aiming at the enemy's weak points. The Pokédex also states that it instinctively hides in holes, which seems indicative of a Pokémon that knows the value of good cover. I suspect most of its victims probably never notice its presence until it's too late.
F.A.B.