To unlock the Postmodern Picasso trophy in RAGE 2, you will first need to obtain the Rocket Launcher from the Strongbox Ark in the Torn Plains region (north of the raceway).
Once you have the Rocket Launcher, you will simply need to kill a total of 200 enemies with it. You can fire the Rocket Launcher in any direction by simply pressing
, or you can lock onto multiple enemies and fire tracking missiles by pressing
first.
You shouldn't have any trouble finding enough enemies to unlock this trophy, but if you want to farm it quickly, you can kill all but one enemy at Bandit Dens, and then reload the last checkpoint. This will keep the progress towards the trophy and respawn all of the enemies for you to kill again.
If you need help with the location, you can watch the video below (credit to Apache on YouTube):