In Mission 1: Post-Human, there are many opportunities to get double kills by shooting explosive barrels. A good place to do this is in the outside area where the attack helicopter comes in as enemy backup, shortly after obtaining the Predator Bow. The last two Cell Operatives will be sitting behind a barricade, and there are two explosive barrels next to them. Shoot the barrels to get an environmental double kill and the "Improviser" achievement.