I used Ivara for this. Very easy, because you can use her Dashwire arrow to create a zipline just about anywhere on the map. Get up on the zipline and use Prowl to go invisible. Even if the enemies are aware of your presence, once you go into Prowl, they become unaware without having to reset the alarm. Then use a Bow or any silenced weapon to get your kills. Pick a map on Earth, so everything is a 1-shot kill.
You can probably use Loki too, but you'll have to find a zipline somewhere on the map.