Story related and can't be missed.
On the third island about halfway through you will be told you need to find some batteries to power a portal so the litterbugs can go home. Once you reach the final area of the island there will be multiple stone platforms to jump across you can grab batteries from. You will then need to take those batteries to the stone faces (they glow and look like the Moai from Easter Island). Shooting dead batteries into these faces will charge them up. Once ten have been charged return to the portal and shoot them into it. The litterbugs will return home, presenting you with the golden key needed to finish the game and your achievement will pop!
This will likely be your second last achievement due to the fact that you will likely need to wait around to be able to finish repaying your loan from the bank.
PS: The stone faces are quite easy to find, just methodically work your way back to the starting area and you should be fine. There is one to the left of the portal and two each in the squirrel area and the starting area. The only other one that is kind of out of the way is to the right of the door you go through to reach the upper half of the island. Halfway in between the door and the slide at the end of the turtle area. The rest are very obvious.