!! This trophy is missable, so be forewarned. !!
After unlocking the C.O.P.S. racing mini-game and getting the Horseman of Death, you can immediately remove the AR goggles and race down the streets of the city, crushing Torture Me Elmer dolls with no rhyme or reason than wanting to do so. However, you will want to do this for the seven decal opportunities that lock this trophy in place.
The status Elmer decals (Bronze, Silver, and Gold) will unlock through repeatedly satisfying the C.O.P.S. demands of destroying Elmer dolls using the AR goggles. The conditions will rack up with every additional challenge, going as far as asking for 60 Elmers in 60 seconds (1:1). For these, the quickest and simplest way would be to coast along the left side of the road on the sidewalk, as that is where the majority of them seem to flock. You can speed your car up by pressing forward on the control stick, but one thing to highly suggest is to steer clear of the jumps. It will make you miss a lot of Elmers at once and you're unable to steer while in the air. (1/7, 2/7, 3/7)
The remaining four are tied to random events within the cityscape:
4/7: There will be a red cheese wheel that will roll in from any of the intersecting streets. Simply run through/over it to get this decal.
5/7: A set of bowling pins will be neatly set up straight in the middle. Line your license plate up with the median lines (to an extent, might require a few runs to get it just right) to knock all ten down.
6/7: A large "Good Tire" blimp will be seen briefly in the sky above the city. Take out your gun and shoot it to get the decal.
7/7: A piano will be dangling precariously from a window on the far right side. Using your gun, you need to shoot it. The timing for this and the blimp have to be precise, as you will miss it often otherwise.
The piano and Good Tire blimp are harder to get to spawn, so if you have the ability to do these, satisfy them first. Your car's speed would help greatly with the piano (slowing down but waiting forever) whereas it doesn't matter for the blimp (though it will be harder trying to land a hit if you get under the steel beams when it appears).
You can check on your progress by quitting out of the challenge and checking the back of your DeSoto (there is a highlighted option for Decals that you can interact with).