Encounter: Dormitories (Chapter 8).
Select the Dormitories encounter from the main menu (chapter 8). This is the graveyard section of chapter 8 after solving the puzzle in the catacombs.
Getting past all enemies unnoticed is pretty easy. The trophy pops when you've opened the gate at the other end of the graveyard. You are not allowed to attack anyone and may not be detected. First, climb around the cliff near the catacomb exit. Then go through the bushes / tall grass. Enemies cannot see you while you're moving in the bushes. You can climb the wall on the right when all enemies have walked away. Now you hear two guards talking to each other in front of the gate. Hang at the back of the wall while they talk. When they're done talking, climb up and hide for about 5 seconds until the third guard walks away. When all 3 guards on the upper area are facing away you open the gate.
If you are being detected simply restart the checkpoint or encounter.