This is the hardest achievement of the game. Remember to dodge, dip, duck, and dive. You'll learn by trial and error what works. Remember that you have rockets and to use them - especially when you have the Choppers with 20 or 40.
For the most part, the game isn't too hard to get five stars on. Especially since you can use a better chopper for the earlier levels once you unlock it. Learn which ones to use the small, fast helicopters and which to use the big slow ones. Levels with the injured people for the most part can be done with both types.
If you're willing to buy DLC, the Albatross and Night Avenger are very helpful for this achievement.
For further assistance, refer to this thread.