I feel your pain siggy. took me many deaths to finally get him on veteran. Best tip I can tell you is to watch him for a bit and see if he is sticking to pattern. Then when you see him at edge of window wait like 3-5 seconds and Hold your breath and line up the shot prior to him sticking his head out. You should have enough breath to make it until he sticks his head leaving you with a nice steady shot at his dome-piece.