Originally Posted by Link2430
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.
Perfect explanation. It's very easy to get too wordy when trying to explain this, and you did a good job of keeping it simple.
I got the Perfect Fight achievement by accident as well.. I actually found this thread searching for a similar topic before I posted mine. WTG.