PLEASE NOTE: The online servers have been shut down since April 2014. Trophy is no longer attainable.
Going for Specter Officer using a variation of the Chicago Method. Unfortunately the provenance of this variation is not known. I am only presenting our application of it here.
Non leveling players select Spec Ops and stand where they continuously experience injury. The leveling player is a Medic healing the SpecOps players (holding down R2 button) while the SpecOps throw their ammo (tapping O button) at the Medic.
There are four start locations on the Chicago Co-op map: Garfield Park,The Museum, The Subway and The Alleys. Instead of The Alleys and the popular burning car method, use the Garfield Park spawn location where you are continuously attacked right at the start by enemies who stay at their location on and under a walk bridge. There is no fighting and nothing to burn out. This will allow everyone to get 80,000 to 99,999xp as the Medic per match since a player can respawn in-match as a different class. Be sure that someone goes down the stairs and comes back up at the beginning to really get the bullets flying - just don't go too close to the enemies who dutifully stay behind their 'line'.
The Medic stands safely behind the red brick column to the right healing the SpecOps while they stay in the line of fire at the top of the stairs. The Medic's Berserker meter in upper right will fill quickly and the "Ring of Life" Berserk should be used just as often (press down of D pad).
A little before the Medic reaches 99,999, the next player should move away from the healing and allow themselves to die to respawn as a Medic (hit triangle and select class) and get ready to take over the healing. Once the switch occurs, the previous Medic dies and changes their class to SpecOps to now throw their ammo to the Medic. Rinse and repeat after one rotation as the xp limit per player per match is 99,999. If you don't get the right location or need to end a match, simply have everyone run forward to be killed.
And because there is plenty of room, you can have eight players (either actual, guest or second PSN accounts) in the match allowing the XP to collect rapidly. 2,200,000 xp is needed for trophy to pop - 22 to 27 rounds.
] Since a turn as Medic to 99,999xp can take up to 10 minutes, I highly recommend sessions of 3 players each with 2nd controllers. A 4th player does not add xp any faster and would only add 10 minutes to a round. A 3 hour session would allow everyone 5 rounds. If 6 or 8 actual players, a 3 hour session would only allow 2 rounds each.
This variation was used by HamsterExAstris
and posted here simply to help others. We are not seeking credit.