There is an easyer way to do this but YOU WILL NEED A SECOND CONTROLLER
Sign a profile in on the second controller and set it to the difficulty easy. Now set the first controller to the one you want the achievement for(MEDIUM or HARD). You enter the level 3-5 playing as the second controller.You wait till the end of the level where you finaly about to kill the boss for good and just when his health bar in nearly empty you press start on the first controller to spawn you into the level. After that just finish him off and you will get the complete game on Medium or hard achievement for the profile signed into the first controller.
I Hope this helps!