When you enter a level, Just play the level for awhile. Try to help your friends and getalot of kills. Then on the screen will show you where the special item is. But alot of times you don't see it because you are to busy or something. So just look in your history and Flip down ental you find it. It will tell you the exect location of the item.
Also try killing all the officers first and leave the last bose for the end. This code works in some leaves not all but must of them.