The Video and guide below was not created by me i am just sharing it with people as this really helped me lvl up in spec ops extremely quick.
FULL CREDIT FOR THE VIDEO GOES TO PREYKOUSIS FROM LIVESTYLEGAMING.
Load up "Fire Mission" and select split screen mode with 2 controllers. Have your main account in the gunner and the guest on the ground not moving. Destroy the first 2 cars with the 40mm rounds, then 2 more to the right. Switch to the 25mm rounds and move to the center and take out the chopper. Pause and restart.
Doing this fast enough can net you 6,000 to 9,000 XP per minute which is 360,000 to 540,000 XP per hour.