Destroy All Humans!

You can get this in the 1st Level after you meet the Farmer. Once he's dead. You need to take his Brain. Rather than holding down . You want to button Mash instead.
VIDEO GUIDE:

Really simple: instead of just holding X on a victim, mash it. The game even promps you to do so.

Here is my 100% Achievement guide and full walkthrough so you don't miss a thing.

So you can get this within the first 5 minutes on Turnipseed farm (Level 1).
We have to do a few things with cows before the farmer comes out looking all pissed off.
Kill him how you see fit (your gun, or throw him in the air).
Go to his body and the game will tell you to mash ..
So.. do it!
Don't hold the button, just keep mashing it and it will unlock..
Technically unmissable.

Go to 5:20 in my part 1 video to see where and how to unlock this

Hope this helps!

Part 1

Part 2