There are two ways to get this achievement :
a)you have a lot of time and patience and just complete this game on insane using a lot of cover and skill
b)you use the cheat which let's you select the final mission, you beat the boss and achievement unlocked.. ok for the cheat you stand between the two rocks at the beginning of the game and hold LT+RT+LB+RB+A until you hear a strange noise. you should be able the select GUN in options. Load the mission Magruder's Mine. Once the mission starts, go back into pause menu, options, Gun and load the last checkpoint of this mission...Once that starts, (this is the important previously missed part) again go into pause menu, options, Gun, and "Toggle Help"...This will now load all the guns and weapons you should have at this point in the game! (This also unlocks the 'all upgrades-achievement') Now just kill Magruder and then insane/hard/normal/easy achievement (whichever you're doing it on) unlocks once you killed him.
Credits to Dad NorthVT and weeken for contributing to this solution.
For a video to see where the right spot is go here :