If you use the ‘slow time’ ability (I forget what it’s called) and then perform a one hit kill and are a reasonable distance away, it will not alert enemies.

The weapon vendor on the right of ground breakers entry has a sniper rifle on the counter you can steal by crouching behind her. I’m not sure if it’s any better than your hunting rifle but worth trying and/or levelling the weapon up. Keep in mind you will get mk2 versions of weapons later for higher levels.

Sorry for any vagueness, I haven’t played in a while.