Primarily, you should complete the game on casual because it is mandatory to complete the game on either hardcore, normal, or casual to unlock insane difficulty.Doing it on casual on your first play through instead of hardcore would save you much more time and you would stack up more achievements when completing the game on insane. Once you have finished your first play through you can begin insane mode. There are two methods to easily complete this. Idea 1 is to have a friend play on casual while you play on insane so you can just sit back and shoot. Idea 2 is that you can play the chapter on casual and once you get to the last checkpoint of the chapter you can switch the difficulty to insane which would mean that you are only required to complete the last part of each chapter.