Engineers are the slow moving, flying creatures that you encounter while playing as the Rookie in New Mombassa. You'll need to kill 10 for this achievement.
If you shoot the Engineers, you'll find yourself out of ammo very quickly. Instead, fire an over-charged plasma pistol shot at it. After a few seconds, the Engineer will explode.
It's recommended that you play through the game without killing any Engineers first for the Good Samaritan achievement. Once the campaign is complete, load up Mombassa Streets and hunt down the Engineers.
Refer to this map for the locations of the Engineers.