Must be done in Co-Op.
This can be done split screen or on Live.
There are four missions for this:
These are missions with check points that you have to go through and finish to complete each. There are also Dead Drops, Laptops and HVTs to capture.
If you are having difficulties with the game glitching out on the last mission, set the difficulty down and do not die. Your failing this mission at that point seems to trigger the glitch. Thanks to azGRIMES for finding this out.