I found a very good place to get this and a lot of the other achievements which require you to grind kills.
When you get to Fallbrook on the planet Monarch, you can find someone called Catherine Malin who will give you the quest ‘Slaughterhouse Clive’.
When you get to the front gates, save before entering.
1. Clear out the few enemies at the front and get the key off the leader.
2. Enter the compound and defeat the enemies in this area..
3. After a couple of rooms you will be in a large room downstairs where they keep all the live cysty pigs. All these pigs count as kills.
4. There’s another room after the pig room with enemies too.
5. Once the place is cleaned out - reload your save. Rinse and repeat.