Wall bang is an attack that penetrates a wall or other surface to hit an enemy on the other side. If you do it successfully you'll notice a symbol next to your name on kill feed.
If you're strictly going for this achievement I've found it easier to do in Team Deathmatch due to near spawn points usually have covers/boxes. Peak, locate an enemy, move back half way behind a cover and shoot. You don't need to be %100 hidden. As long as your shots are going through a cover results a kill, will count as wall bang kills.