This is an achievement that will take some time to unlock. Across a single playthrough you must kill 2000 enemies. Not to worry, there are plenty of enemies in the game. If you have not completed this before completion of the game or reaching rank 20, simply find a location to farm kills. To track your progress, go to leaderboards, then select "Justice Dispensed" and filter it to "My Score."
There are quite a few chickens and other enemies if you teleport to the Rainbow's End outhouse (21 on the map) and walk to the Casino. You can also hit all the small islands and maybe the Orque fortress, then repeat.