You need to kill all three bucks or bulls on all 15 sites of an adventure. This is most easily done in the Elk Adventure (Main Menu>3 Treks>Elk), as they are bigger and slower. This achievement should not be that difficult in theory, but because of the shoddy controls, it becomes quite a challenge not to miss a single one out of 45.
Fortunately there exists a method to make it easy to obtain. Any time you miss a bull (i.e. it runs off screen), instantly push the Home button on your phone. This will send you back to the Start Screen. Launch the game through it's icon or the Games Hub (don't just push back) and it will boot back up to the Main Menu of the game. From here, click on the "3 Treks" icon and then "Resume Trek." The game will load back up to the beginning of the level you were last on, with your miss having no impact on the achievement. You need to be quick with the button push however, especially if it is the third bull you missed. If it gets too far it will auto-save. Remember, for this achievement you main goal is just to kill every bull quickly, not precisely. Line up the first bull which is stationary with a good shot, then just rapid fire the other two trying to hit them twice each which will kill them.