If you want to do PS on your own then you must first do ALL of Gustav's quests.
It seems long, but actually it isn't all that much. arrow She Wolf, Recipe items, Papa Saw
It's just annoying that you have to fight a lot of monsters to get all of the recipe items. **
But after that he will tell you to kill Papa Saw.
Once you did, you have unlocked PS forever, hooray for you !!! xD
** NOTE: the recipe items are found in different areas. Make sure you get them in the correct area. Buying them from the Marketplace doesn't work. ( i tried that xD )