Stun an enemy by dumping sand on their head. (campaign only)
You will get this achievement while playing through the game, and it's hard to miss. There are multiple ways to get it, but all you'll need to do is trigger some piled up sand, causing it to spill onto an enemy.
Chances are that you will unlock this achievement randomly when shooting at enemies and using explosives, although in case you don't have it, there is a spot where it is almost guaranteed. While playing Chapter 10, there will be a point where your team mates are trying to unlock a door, and you will have to hold out the position for them. The room will be now stormed with enemies. Wait a few seconds for enemies to fill the room, and now shoot the glass pane on the ceiling, letting sand fall onto them. This will unlock the achievement for sure, if you were unable to get it while playing.
The easiest part to get this is in Chapter 10 "Riggs". Just before you get to the aquatic place you have to hold off some solders while Riggs unlocks the door. In this room there are two glass cealing pains. Wait for the solders to come in and then shoot the glass. There were 3 of them in the room when I got this.
Just got this by accident. Chapter 7, during the fight after Gould tells you to escape your squadmate yells "They've got an HMG." I did a blind grenade toss in the general direction of the turret and it popped.