Select the first Spec. Ops mission "Stay Sharp" (the obstacle course) and set the difficulty to Hardened or Veteran. Just complete the mission to unlock the trophy. If needed, you can also get the "50/50" trophy during this game:
Plug in a second controller and choose splitscreen. All you have to do is hit 17 targets with each controller and presto, 2 trophies for the price of 1!