First of all, take a look at the help & options, it's really useful.
Take and hold a pygmy and get him in the sky. Drop him on a cloud, he will bounce in the air. Now bounce on the other clouds trying to go as high as you can. There are several type of clouds. Some clouds disappear (those clouds have a little more grey in them), some clouds make you do a big jump (dark grey storm clouds,) some clouds make you do a low jump (snowing clouds). There are also enemies shooting at you at some points when you go higher and higher. Once you reach the required score (6000), just fall all the way down and you'll unlock the achievement.