Once you're loaded in, Pause the game and drop the difficulty down to Hardcore. You'll be right at the moment where you have to destroy Victor's plane. Boot the game back up and select Continue from the Main Menu. Once that pops up, press the guide button and quit to the Dashboard. Upon destroying Victor Branco's plane (and ending the game / triggering the final cinematic), you will be awarded with the Achievement for Old School. If you don't WANT to go through both Hardcore AND Old School, then here's what you do: Personally, I found playing Hardcore substantially more difficult than Old School (which I find ironic, since the achievement for Old School is of a higher point value).