This trophy is very easy to unlock if you know the exact location. It can be unlocked at any time and with any character. To get it visit the UNX Training Camp, Hawaii and replay the training mission.
At one point you will be asked to melee one of your enemies. Instead of killing him with your knife, stay in front of him and start shooting. He will continue to respawn until you complete the objective. So start counting the bullets consumed. It should take around 10 minutes. Now, complete the objectives and the mission.