This is more of an updated solution for the achievement, considering ive just completed the game i thought i would upload the video of me finishing the game on Insane to see if i could help anyone. I would recommend completing the game to 100% before attempting this (it will make it 100% easier because you can carry over your weapons).
I have created a video of me doing the cheat with instructions which i have linked below, this can be done on every difficulty if need be, it doesnt just work for insane.