Spawn at the Agency tower and take out the Harpoon Gun. Ride out to where the Peacekeepers and the Los Muertos are doing a shoot out. Kill all the Los Muertos members. Pick up their bodies one at a time and throw them on top of a car. Then, quickly shoot them with the Harpoon Gun. This may take some time because the bodies will slide off and some may just fall even if you harpooned them. Just throw them back up on the car. When you Harpoon five bodies on a single vehicle you will get the "Body Armor" achievement.