Parry times Guard as an attack arrives. Deflect uses held Guard plus a timed additional command in the method covered here. Deflect requires its own skill unlock and focuses on enemy stamina; Parry usually creates an opening. Parries also help charge Blaze once Blazing State is unlocked.
In a Story or Action run, use a Spirit Mirror and select Change Difficulty Level, choose your mode, then use Confirm. Story is the easier option. You can switch in either direction and change back later in the same run.
Onimusha: Way of the Sword guide hub. Browse game settings, combat guides. Available guides include Parry vs Deflect in Onimusha: Way of the Sword: Inputs and Outcomes, How to Change Difficulty in Onimusha: Way of the Sword, with requirements, routes and encounter notes.