To get this, you will need a sniper rifle, or the scope mods for Assault Rifle or Pistol. Look for a sniper rifle with a high rate of fire, or if you have the mods, an assault rifle with high accuracy, or a Pistol with both.
Aim for the head on humanoid enemies, and the lights on robotic enemies. Animals have varying weak points.
You have a chance to pick up a sniper rifle on the prologue. It's next the ship Cora crashes in. After the prologue, you'll have access to the pathfinder sniper as well. During Eos, scope mods for Assault Rifle or Pistol could drop. If they don't, new vendors will appear at the Nexus. The 'General' merchant should always sell both scope mods.
Good early weapons to use are:
The pathfinder assault rifle: high-accuracy 3-round burst. Increase stability for best results. Can function like a sniper rifle.
The Avenger Assault rifle: Low-accuracy, but high rate of fire. Any scope will boost accuracy, but it's still a medium range weapon.
The viper sniper rifle: Better rate of fire, lower damage per shot than the pathfinder one.
Later on, try:
P. A. W. assault rifle: This is my favorite weapon. It fires a continuous beam with near perfect accuracy. Give it a scope and stability upgrades.
Raptor Sniper rifle: The highest rate of fire and stability of any sniper rifle. Functions like an assault rifle. You may want to equip a spare clip mod.