This achievement is quite tricky as the range on the green bird's scream which scares the pigs is not that large, meaning you have to use it very close to all 6 pigs to be able to unlock this achievement. The best level to earn this on is Level 2-18. Use your first green bird to knock some of the stone blocks off the top of the largest, middle tower. They need to fall in such a way that none of the pigs are killed. Then, with your second green bird, basically just fire it the same location so that it arcs in between the middle tower and the farthest tower (the stones you knocked down with the first shot makes this possible). Right before he makes any impact and you are near the two groups of 3 pigs, tap the screen to make the bird scream. If done correctly, you will unlock the achievement right away. Thanks to DarkxMaterials on TA for this method.