At the end of Chapter 9 when you are about to go into the University, you have 2 options
1) go to the right side where 2 guys come walking out, making it super easy and you can let them live or kill them, up to you
or
2) go towards the main door that has 2 shielded and helmeted guys who in front of the main door next to the carriage
Go to #2 the main door
I personally used a pot to lure one away and then sneaked up to the door past the 2 guy and clicked on the door and BOOM achievement.