1. Enter the rightmost door of the monastery.

2. Next, you need to cross the courtyard and enter the corridor, as shown in the figure below.

3. Clean up the guards at the door, and then move on, as shown in the figure below.

4. Next, you need to enter the hall to kill the BOSS inside, as shown in the figure below.

5. After killing BOSS, find Duane's safe.

6. Finally, you need to get the key, as shown in the figure below.