This is completed in the fifteenth mission "Heat". At the beginning of the mission, you will be defending a position at a church. Head up the tower in the church and you will find a cache of RPGs. Now just use them all up, you do not need to kill anyone with the shots, just take the shots. 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...