Here's a quick method that worked for me: choose Mombasa Streets mission on easy and go to the supply cache to get a Mongoose. Then go south in the direction of uplift reserve - just before it there's a crossroad with an engineer flying on your right (easy to spot in VISR mode). Kill it and go back to the supply cache, then return - the engineer should be there again. Repeat this until the achievement pops up.
*Sometimes the engineer will not reappear, in this case just go elsewhere and it should respawn again.