Kill 1,000 enemies in "Jimmy's Vendetta."
You probably won't get this through normal playthrough of the DLC. On average you will only accumulate 300-400 kills per game. You can check your progress by opening the Game Statistics from the pause menu where it's listed under "Enemies killed." If you're going for the Millionaire achievement legitimately, you could possibly get this while replaying missions to increase your score.
The counter is carried over from one playthrough to the next, so you can either replay the DLC or repeat missions to get the extra kills. Missions like The Don have enemies that continually respawn so you can replay this or any other as often as needed.