Kill 300 human enemies.

Human enemies will be present throughout the entire game and there will be plenty opportunity to kill them. However, bear in mind that NOT killing humans is a requirement for "Duke", "Damir", "Alyosha" and subsequently "Full Strength" so if you're attempting to unlock these during a play through, you likely won't kill many humans.

If you plan to do two full playthroughs then feel free to go 'loud' and kill everyone you can during one of them. If you plan on using the Ranger Hardcore glitch to circumvent a second play through you still may need to grind out kills, though probably not many.