Leo's Fortune

Hardcore is unlocked by completing the game normally once. After completing the game, on the main menu in the upper right corner you can attempt the game in hardcore mode for this achievement.

Hardcore mode tasks you with completing the game from start to finish in one sitting without dying. If you die or quit out, you have to start over again from level 1. I think it goes without saying that this is by far the hardest achievement in the game and the reason this game is a 9/10 for completion difficulty. After spending some time trying to finish each individual level without dying, you now have to finish the entire game without dying. Unfortunately there aren't any exploits or workarounds for this achievement - no copying saves, dashboarding, etc. This one purely relies on skill.

The biggest thing I can recommend is practice. After you replayed all of the levels to get the zero fatalities stars, you should have a good idea of which level gave you problems and which levels you just made stupid mistakes on. In particular, you should be very comfortable with the first two acts since those are the easiest. Do a run through each chapter in act 3 just to make sure you're comfortable with it. Acts 4 and 5 have some really dangerous parts that are really easy to die at. Couple that with the fact that you'll probably be anxious and on edge if and when you actually make it to act 4 or 5 during a hardcore run and your likelihood of choking and making a mistake during these levels is significantly higher. Practice the levels in acts 4 and 5 until you aren't worried about each obstacle anymore. Then you are ready to try tackling hardcore mode.

I have provided a video below just for completeness sake. It may help some people to see it being done, but you really just need to practice each part so you can do it consistently, especially since watching someone do it makes it look easy. Full credit for the video to its creator.