To unlock The hard way trophy in A Plague Tale: Innocence, you will have to enter through the main door of the University in Chapter 9, "In the Shadow of Ramparts".
When you reach the University, there are two doors you can use to enter: The main entrance which has two enemies guarding it, and an unguarded side door. For the trophy, you will need to enter using the main entrance.
Guarding the door, you will find one enemy with a shield and one with a helmet. You can find a simple method of how to kill them below (credit to PSTHC on YouTube);