There are two save files. 1. Insane Difficulty completion without changing the difficulty. You just need to play the final part of the final boss fight and achievement will pop. 2. 499 disc derez kills. just kill derez one more enemy and achievement will pop. The above two will save you half of the time in completing the game. Don not use these save until you have completed all other achievements excepts the above two or you will mess up with your achievement order.
