Plug in a second controller and choose splitscreen. Select the first Spec. Ops mission "Stay Sharp" (the obstacle course). All you need to do is hit 17 targets with each controller. Civilian targets won't affect the trophy, check your score by pressing Up on the D-pad. Just take one controller, get to 17, take the other controller and finish the course. If needed, you can also get the "No Assistance Required" trophy during this game:
Just set the difficulty to Hardened or Veteran and presto, 2 trophies for the price of 1!