This Achievement is easy in thought, but in practice is mentally challenging. you monster
To get this Achievement you must kill 1000 animals including pigeons, seagulls, crows, or Father Grigori for some reason.
You can spawn many pigeons and crows in the same spot, as they are small and don't cause much lag. Once you have a large cluster of 100-200, you can blow them all up at once with an explosive. Repeat several times for the achievement.