Man or AnimalG 30

In the level Bad Blood, you'll enter a facility and a lot of people grind out this achievement right after getting in, but I found it to be better after your first cut scene you'll go down a small incline and three guys will come running from the left. Quickly hide to your right and one soldier will walk past for an easy stealth kill, then turn the corner to the right where another soldier will be set up for a stealth kill.

SAVE and reload. You must save to have those two kills count. Yes that means 50 reloads, but it's fast and reliable and right after a checkpoint. That's why I like this spot.