This is one of the trickier achievements in the game, not only because of the poor hit detection of the controls, but also because of the luck that is involved in killing bucks/bulls with a single shot. You basically need to kill all three bucks/bulls in a level only firing a total of three shots. You must get single-shot kills on all of them and not miss at all. It is easier to do this on the Elk treks, as they are larger animals and seem to move slower. The British Columbia location seems to be the easiest of the elk levels.
Make sure you perfect shot the first elk, which is usually on the screen when you load into the level standing still. Then get the next two with single-shots as they run across the screen. Just remember the kill zones and aim for them. Do not worry too much about this one as it will probably come easily when you are playing the levels over and over to get your 500 kills.