15 Sep 2011 12 Oct 2011
UPDATE: Solution completely remade, found a way to earn this achievement in about 30 minutes. See my attached video.
In short, load up Act 2 Chapter 2 with 2 controllers plugged in, make sure you put one of the characters as far up the last hill, as close to the door as possible when the checkpoint at the very end of the chapter occurs. Finish the chapter, pause during the cutscene and reload checkpoint.
Run up the hill and finish the chapter again and just keep repeating.