I'm xbox, otherwise I might help ya.
I got the doors achievement done on custom/easy. The only real difficult spot, I found, was at Farm opening up the house (I went in the back door and out the front). Town, despite all of the lava, wasn't really too big a task. Obviously, be sure to hit all the turbine doors (including the second/back door of shed @ Farm; that one doesn't always open w/ turbine at the front) & PaP, and both bank vault doors... If you do find someone to help ya--- give them money from yer bank, maybe in a separate game. They can open all the doors while you ride the bus in peace and you'll get the trophy.