To unlock the Dark Zone: Takedown trophy in The Division 2, you will need to kill a disavowed agent in all three of the game's Dark Zones.
For a player to become Disavowed, they will first need to turn Rogue and then kill a player to become disavowed. Once Disavowed, they will be highlighted by a red skull.
A simple boosting method for this trophy. Enter a Dark Zone with a friend. Have them become Rogue by pressing the touchpad. Get your friend to kill you so that they become Disavowed. When you respawn, kill them. You will need to be far enough away from a Dark Zone entrance so that the automatic turrets down kill them as soon as they go Disavowed.
It's worth noting that you can only do two Dark Zones in a single 24-hour period. Every day, one of the Dark Zones will be the Occupied Dark Zone, which will stop agents from becoming Disavowed. The Occupied Dark Zone will be highlighted on your map with the red dagger symbol.