You are going to have to complete all 22 levels with all the boys.Yeah it will take a while.I HIGHLY recommend getting all secrets and mega men with each boy that way you wont have to complete them again.Not really a guide but more of a heads up.
Here's a link to all secrets.Also mega men WILL be found and time cores.And of course secret areas.http://www.xbox360achievements.org/forum/showthread.php?p=49...
EDIT:Now with the patch,you can enter a level with any boy any change them by pressing the "Back" button.(Left side of the Guide button)So let's say kenny is your best person and you have to complete it as cartman. Just press the back button,select him,and complete the level.
Remember 22x4=88.Which means that's how many levels you'll have to complete.It's not hard anymore but if you actual die after the patch,(you suck) but i recommend getting a partner.Have Fun! (HA)