As you progress through Mission 1: Baku, you will come to a small crate marked by a "gear" symbol on your map. Make sure you equip the C4 at this point. In the first section, you can mark the targets, and sneak around to kill all 4 of them with 3 C4 charges, although it's not completely necessary that you kill all of them. Now, after killing all of them, immediately sprint forward towards the objective where a truck with 3 enemies will stop. As soon as it stops, throw a C4 charge and detonate when they jump out to get another 3-4 kills.
From here, I suggest you just progress as normal until you reach the next equipment cache later on, where you can refill your C4, and come back to it at any point for extra ammo. What I recommend is first sprinting up the scaffolding on your left hand side after exiting the pipe. There should be 3 enemies on this catwalk that you should be able to kill with 1 C4 charge, and you will unlock the trophy here if you've not missed any enemies. Otherwise, sprint towards the middle of the area, and kill some enemies near the truck, and sprint to the very back of this open battlefield to find 6-8 enemies all bunched up together, and you should be able to kill most of them with 1 or 2 C4 charges, unlocking the achievement.
View the video below for a visual and more information: