Target the weak points on structures to take out several pigs at a time. Also focus on TNT crates to send the well armored pigs falling. Make shooting flying pigs a priority. They are persistent and can cause lots of damage. Do not ignore pigs in the background. Watch the horizon to make sure you find the distant pigs. Occasionally pigs will be hidden deep within structures. Shoot out glass blocks, and other points of entry to expose them. Keep shooting, even though Astrotrain is in sight at the end of the level. You can still damage pigs until you are inside.