This is completed in the fifteenth mission "Heat". After defending the church, you will be on a machien gun attached to a downed helicopter. While in this position, five helicopters will fly past, be sure to take them down quickly. Then, toward the end of the mission you will be making your way to a barn. Half way across the field will be three RPGs, be sure to take all three shots, turn back around and take out the three helicopters before they leave. This video was recorded on recruit difficulty, as the difficulty level does not affect the achievement / trophy.
Here is a video guide with English commentary for this trophy!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUJ3mDD5Tj6n...