All bucks/bulls across all adventures, treks, and multiplayer count towards these achievements. You need to reach a total of 500. The fastest way I found to grind these was to play 1-Trek Elk on British Columbia in 4-player multiplayer mode. Elk are easier to hit then deer and British Columbia is the easiest elk location. Also, playing in 4-player mode reduces loading times and will help you learn the levels as you play them. Depending on your shooting skill, you can earn between 200 and 300 kills per hour with this strategy. After you earn all of the miscellaneous achievements, you will likely still need 300 or more kills. Remember accuracy is not important here. While grinding, focus on rapidly firing at the elk to hit each of them twice to kill them.