Found at the end of Mission 15: Ghost Team near the end of the level you'll climb over a wall into an arena with a tank. Ignore the objectives and just concentrate on knocking over the 33 targets. If you ignore collecting the C4 no further hostiles will spawn, leaving you with just the tank.
Now move round the arena in a circle clearing building by building all the targets, checking both the ground floors and the roof. While constantly keeping out of line of sight with the tank. As if you die, you'll have to start them all over again (isn't any checkpoints). If you find you're missing one just retrace your steps in the opposite direction and it should turn up.
See my video below for the locations of all 33 targets. My last missing target was right in front of me at the end - can you shoot that one for me please! :D