Black Tusk Grenadiers have pinpoint accuracy. This is the case for Reds, Veterans and Elites. They can hit your location from across the mission map, and dont require line of sight.
Please see video of several occurrences, below
https://youtu.be/G4ygDPe_ANk
Please see video of several occurrences, below
https://youtu.be/G4ygDPe_ANk