After you level Aloy up, you will have enough skill points to unlock skills.
Stealth Strike is one of the first ones you can unlock.
Buy it with 1 skill point.
And now when you hide in tall grass or sneak up behind a machine, a prompt for that skill will appear.
Activate that skill and stealth strike your target.
Watchers are the ones that can be killed in one strike so easiest way is to stealth strike 10 Watchers and you should see this trophy pop.