REALISM ACHIEVEMENT GUIDE:
No video attached unfortunately, but work with me here.
Mission 4: Proxy War
Achievement: Ashes to Ashes
Once you breach the outer wall of the base, move through the mission as normal. Around the halfway point through the base, you'll see an alley of storage crate buildings on the far right side of the base. Clear that side out, and use the RC plane to take out the snipers off in the distance. DO NOT MOVE PAST THE STORAGE CRATES UNTIL YOU ARE READY.
Once you've prepared, step out past the storage crates towards the road paths that are leading out of the last section of the base. Once you do that, a troop carrier truck and an all-terrain truck will spawn. The all-terrain will ALWAYS have 4 soldiers spawn inside of it. Stay in cover and throw a molotov at it. They will all burn alive before they can exit the vehicle, unlocking the achievement.
I find this method to be more useful than the "lucky chance" of throwing it at the soldiers the helicopter drops off, or the "berserker" method of jumping into a crowd in the second act of the mission.