Destroy All Humans!

This achievement is earned during the mission "Alien Pool Party" or any mission thereafter. To get this, simply target an object and press the X button. A portal will appear above the object and turn it into ammo. Upon doing this, the achievement will pop!

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

On level 4 (Alien Pool Party) we get told we can now target items and turn it into ammo!
Nice!

To do that, target any small item (bin, umbrella, chair etc) hit the button a few times and a small portal appears transforming that item into ammo.

Be warned that the ammo produced can be random through the game though.

Go to 42:55 in my part 1 video to see where and how to unlock this

Hope this helps!

Part 1

Part 2