The Druganov is the sniper rifle in Rogue Warrior. A great place to farm this achievement is in the first level. You will come to a window with an enemy in front of it, take out the enemy and there will be a Druganov. Look down through the window and you will see a courtyard housing 4 enemies. Take out the enemies with the gun, go down into the courtyard and kill the 4 more enemies that break down the door. Reload your checkpoint until you net the achievement.