Kill an enemy with the RPG in 'Dam'.
***Possible Spoilers Below***
This achievement is pretty easy. After you take the photographs of the helicopter two more guards will come. Kill both of them. Open the crate near the gate they came through to get a sniper, if you don't have one. Kill the 3-4 guys on your right (don't run down to the right, just kill the ones that are there now) as you pass through the gate. Then snipe the guy in the tower on the right side. Now run the opposite direction (left out of the gate). No one will be in this area so it is an easy run. Run up to the top of the tower and on the left side there will be an RPG (the one that blew your truck up earlier in the mission). Now run back the way you were supposed to go (to the right out of the gate). As you get closer to the second tower some more guys will spawn. Now just take aim and blow one up and the achievement is yours. Personally I shot one at fairly close range (just aim at their feet), but be careful as you could probably kill yourself.