This is a short-cut guide based off HonoraryDecoy's solution with a easier to follow format and a video showing each step for those visual learners, like myself, who were having trouble figuring it out. So, kudos to HonoraryDecoy for initially figuring this method out!
Using this guide you will need to play through each of the 11 levels on Cadet difficulty and repeating only the last checkpoint for each level on Sniper Elite difficulty. Having that said, it is possible to do this on your first playthrough. And, all of the levels are incredibly easy from the last c.p. except for one (Kaiser-Friedrich Museum). The rest of the levels you may have to shoot a couple of people or just take cover and run to the end zone.
Here's the link for the video step by step guide:
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1) From the main menu, select "
single player" "
mission select" "
level ???" "
cadet".
2) Play through the whole level till you reach the final c.p.. Then press "
select" & "
exit game" . ***Note: if you completed the level, just wait till you get the completed stats screen and press "
exit game" (do not press continue). You should now be back at the main menu.
3) Select "
mission select" "
same level" "
sniper elite". After the mission starts: press "
select button" & "
exit game". You are now back at the main menu.
4) Select "
single player" & "
continue game". Immediately (before the objective screen has time to load) Press the "
Xbox button" & "
Y" to dashboard out. You are now at the Xbox home screen.
5) Reload the game, select "
single player" & "
continue game". On the left hand side of the screen you should notice the level & sniper elite difficulty. When the game starts after the objective screen, you will start at the final c.p. for that level.
6) Rinse & repeat the process for the next level.