Chapter 10 - Section "Shadow Fall"
For this hidden trophy, you need to kill as many enemies in the last chapter as you can. The problem with this is, that you have a limited period of time and that's what makes this trophy a little harder. After entering the outside area you will only have 3 minutes and 30 seconds to complete the mission. There are 5 enemies in the building where the mission starts. So you still have to kill 15 enemies outside. Make sure you set the difficulty to easy! Look for a position that is easy to defend, but keeps most enemies in your line of sight. The platform where you pick up the sniper rifle (mission objective) is a great spot. You can shoot through the hole in the floor to hit enemies below. Most enemies will move towards your position, so you just need to defend yourself. By holding down on the D-Pad, you can highlight nearby enemies.
The trophy pops when the credits start rolling.