To earn this trophy you need to get through the training area known as "The Pit" with a finishing time of less than 30 seconds. You'll find The Pit at the end of the first single player mission in the game (S. S. D. D.) and in Spec-Ops, but for this trophy you'll be working in S. S. D. D. 

The Pit is a simulation, so there are no real enemies, only cardboard cutouts of enemies and civilians. You will be penalized for shooting a civilian or missing an enemy but, you can get a bonus for accuracy. The best way to get a good accuracy bonus is to get good with pistols and use two of them, since they only fire one bullet at a time. If you're just not comfortable with pistols, it can be done with the semi-automatic rifles at your disposal as well - just make sure you are very light on the trigger.

The key to this is to get the pattern down and keep your eyes open so that you don't shoot the wrong cutout. If you get the timing down right, you should be able to switch weapons after taking down the last enemy inside the house, as you're walking up the stairs, so that you don't lose any time.
 

Pit Boss