Originally Posted by spartancell
so wait, all i have to do is beat the last level on hard? and basically what you are saying is that i can put my profile on hard, and have a dummy profile (profile 1)on easy, and it if i join in later, then finish the level i get the hard achievement??
You're pretty much right. But to clarify:
The easiest way to do this is set Player 1 to a dummy profile and put the difficulty on easy. Next, sign in to your profile for Player 2 and set the difficulty to hard or medium, whichever you need.
Play through Level 3-5 with Player 1 (dummy profile) on easy and when the boss has very little health left press start on Player 2 (your profile) and finish him off.
This is an easy to way to also get the Perfect achievement.