You'll have many chances to unlock this trophy throughout the game, but you can unlock it as early as the first mission.
However, you'll be able to get it just a couple of levels later with a bit more ease. On the "The Dark Within" mission, you'll come to your first encounter with the Hive. Even if you don't unlock it the first time through, you can return here later with better equipment to unlock this pretty easily.
It can also be done pretty easily at the end of the mission "The Last Array". You'll be attacked by waves of Hive. The Thrall are pretty easy targets.
Or you could forget all of that as you'll almost certainly just unlock it through regular play.