This is a cumulative achievement that is awarded for killing 300 humanoid enemies.
That means Va'run, Spacers, Ecliptics, etc.
These kills have to be on foot (no ship) and you can check how many you have by pressing the menu button and then Y.
It should be the first item under Combat.
Also something important to keep in mind is that NG+ does not reset this so feel free to start NG+ if you want.
As for how to get that many kills, it's not a lot.
I played through the story twice (once full and second abridged) and got it without almost any side content.
Make sure to get credit for the kills, though.
If you have an active companion one way to do this is to use a corrosive weapon or anything that does damage over time so they won't steal your kills as much.