If you want to grab some easy XP in the co-op mode, then first off you need to unlock the last mission, which is called "Eleventh Hour".

Play through the level on the easy difficulty and make it through to the part where you must disarm the bomb. Purposely fail to disarm it and fail the mission.

You will still get around 5,000 XP for doing (and failing) the mission and since getting up to the disarm part and failing it each time only takes a few minutes, you can rack up the XP very quickly!