During Mission 3: Uprising, you will make your way out of a garage (shortly after getting the "Roadkill" achievement). Make your way to the bus. You will now be hit by an RPG, as a mandatory mid-game event. Make sure to get up and kill the 3 enemies, that are nearby. Now, you are to leave the bus and kill both of the other enemies before the building collapses onto them. I recommend throwing a grenade between them, and then trying your best to shoot them. If the building does collapse and you do not get the achievement, make sure to reload your last checkpoint to start at the garage again and try again.
There is a possibility to shoot 1 of the 2 enemies before you exit the bus and trigger the building's collapse. You should be able to see their head poking over one of the rocks.