After comoleting Rockwell.
You can go back to Turnipseed to complete the Rampage mission.
This is super easy. Go right to the back of the area (where the farmhand housing is it has a probe on its roof) and use PK to throw the farmhands into the air. Use your Zap-o-matic on ones that get way too close. Use Pk to throw them in to one another too. Normally resulting in killing both, although it doesn't always happen.
Obviously, 3 stars will get the trophy.