This achievo sounds simple enough......
The problem I had was that the goose never wanted to be fed when there were enemies nearby, and he needs to be fed in order to do a poop.
If you have gone through the whole game tying to do this to no avail, then hopefully this will help:
This is how I did it:
DAY 6.
Get on your bike but do not go where the game wants you to go, but instead, leave the trailer park and go to just below the entrance to the long swishy corn field (where you need to go to get the "armour" achievo).
Just below the gate entrance to this area, there should be some golf ball enemies hanging around. Do not attack them. Ignore them for now. They need to remain passive so you can feed your goose and he stays relaxed enough to poop a few moments afterwards!
Press your MENU button, and you hopefully have a "LOAD AUTOSAVE 0 min ago", so "EXIT TO MAIN MENU" , then "LOAD an old part" and, on your last save click "Y" and COPY.
You can now reload the game from here over and over until you get this CRAP achievo!
If you can't, or don't want to create a new save here, don't worry - it just means you will have to start from the beginning of the whole level if you MESS up.......
Load your new save file that you just created.
Now just wait here for a while..... do nothing until your goose wants to be fed . . . Then feed him . . . . Then wait for him to poop. While you wait, switch to Demelza and get your boots ready.
After he has performed his duty, move around the area and you will awaken a few golf balls and they will spring into life and turn into furry toothy enemies.
Do not attack them (because your AI party members might kill them all), just get them to follow you over to the poop and KICK the POOP with your BOOTS (as Demelza).
Achievo will pop after a few seconds if you did this correctly.
If you kill all the enemies but no achievo, exit to menu and reload your save again.
Good luck! Phew! What a stinker!
Here is a little video showing the location (mostly me standing around waiting for the goose) :