NOTE - For this trick to work, you will need two controllers. You will also need to be at least level 15 before doing this, as well, because you need a Tier 4 map. Go to Special Ops, then choose Split Screen (you can leave the second controller as a Guest account). Choose Missions and the one you want to pick is the "Fire Mission" mission. Difficulty doesn't seem to matter, but to be on the safe side, put your main account's difficulty to Veteran; you might want to put the Guest's difficulty to Regular because of it being open to attacks quite easily. Also, make sure you set the main account to be in the role of the AC-130 gunner. The basic idea for this trick is to use the AC-130 to destroy two trucks in the beginning, then two hidden jeeps and the helicopter that comes by; after you do that, you will restart the mission and keep doing that over and over to earn some fast and easy XP for Spec Ops modes. Switch to the 40mm rounds and look for two trucks towards the top left (they should come into view at the beginning), then look for two jeep that are kind of hidden down and to the right from the trucks. Once you take those four out, switch to the 25mm rounds and you should see a helicopter fly into the middle area of the map; take it out and then restart the mission to do it all over again. The XP you earn will be saved on your main account and you can get around 6-9k in about a minute.