I found a very good place to get this and a lot of the other trophies 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.
(For this achievement, you can make another save just before the pig room if you want to speed things up. However, if you are planning to grind other kills, then make a save outside the front gates.)
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 or run straight through and into the main building.
3. After a couple of rooms you will be in a large room downstairs where they keep all the live cysty pigs.
4. Go into crouch and simply hit them all one by one with a melee weapon. These should all count as stealth kills.
5. Reload your save. Rinse and repeat.