Climb the asylum tower without alerting any nurses.
In Chapter 6, check out the map for the tallest building, thats the Asylum Tower.
There will be 3 nurses around this tower, you can avoid the 1st one easily. On the 2nd nurse Just grab a "Zzz" from her, and set that on herself. Than climb to the next floor (stay around the corner, avoiding the 3rd one), and wait till the 2nd nurse wake up, and re-grab "Zzz" from her and put it on the 3rd nurse on the left side. Easily done.
In case you made a mistake, or being alerted, just restart from the checkpoint on the first floor.