The Gunstringer

Just after the first mission, you have three cows on the screen.
Simply shoot at them before they hit the ground.
You can lose 2 cows and concentrate on the last 12x to win the achievement.

Here you can win too :
- Thunderstruck : Shoot the same cow 30x.
-Nick of time : I recommend to let only one cow. Juggle with it during 30s. You win Thunderstruck necessarily.