Originally Posted by SoaringEagle
how do you switch the difficulty so you can get the achievement?
ok this took me a little while of searching but here is an updated link as well as some tips. first off, when you get to the start and put in the code if it doesn't work the first time restart the game and go back to the start. this worked for me. When you go to the start screen then go to options screen to find "gun" there. once this is done quit the game and start a new one on whatever difficulty you are working on. Once through the cutscenes go back to "gun" pick missions and then magruders mine. once it loads pause go back to "gun" and pick checkpoint kill magruder 2.
This was an easy 230 GS for normal hard and insane. Hope this helps. Here is the link