Start up a game of Keys to the City. Hold the back button to open up your menu. Use your DPad to navigate. Click Lb on agent skills. Max them out. Go back. You can even upgrade abilities if you want. Improve health and shield Regen. Click LB when you’re on “spawn objects.” Go down to fun props. Spawn a bunch of beach balls. Go back and click on spawn ai. Click on spawn logistics ai. Spawn a bunch of wardens. Press X to pickup a beach ball and throw it at the wardens. Keep doing this until the achievement pops.
If you're having any trouble with this. when you open up the main menu, scroll down to "cease fire." If you turn that on, enemies won't shoot you and typically stand still.