Get 15 kills with RPG in the Singapore mission in the Campaign.(20G)

This achievement takes place during Mission 4: Singapore. After progressing through the mission, and getting access to a tank, you'll come to a equipment cache, marked by a gear icon on your map.

Get out of your tank and make sure to equip the "RPG-7V2". From this point, you have a lot of different opportunities to get 15 kills. You'll want to keep your tank with you, so continue through the map inside of it, and get out when you see enemies. Progress into the upcoming section, and make sure to destroy all 4 tanks in this area, which will get you an easy 8 kills for the achievement. You are free to revisit the cache to refill your RPG ammo at any point. There will be more and more tanks after this section, but they will take 4+ RPG shots, which aren't really worth your ammo. Once you reach the parking garage, try taking care of as many ground units as possible using the RPG. You might get the achievement at this point, but if you don't, don't worry, there are plenty of enemies further on. You will eventually come to a large building with 3-4 enemies inside, and then a bridge with an additional 15+ enemies on it. Just make sure you bring full ammo to this section, and you should be able to get a large amount of them.

If you miss too many shots, feel free to reload the checkpoint.

Once you get 15 kills, you'll unlock the achievement.