This is a very easy achievement to get. You will have access to the first 3 levels from the start. You will have to keep replaying these to improve your gear to get all the medals. I got this achievement pretty quickly. Just mission 1 already have 7 people to rescue, 2 and 3 have a bit more from what I recall. To rescue people, just fly over them for a few seconds until you get the +1000 bonus.