So Postmodern Picasso calls for a lot of kills with the rocket launcher (found in the Strong Box Ark
in the Torn Plains
). Fortunately, there is one type of location that makes this much easier, mutie nests. They have a tendency to bunch up when they chase you, and destroying a couple of their eggs can have as many as 15-20 spawn at a time. If you happen to have run through all of the mutie nests, they've added a new feature where you can actually replay goon dens to your hearts content, allowing for unlimited kill farming. This is also good for this achievement.
Skip to 2:38 for Postmodern Picasso