Mortal Shell 2 features many unique achievements, including Peter's Perfect Parry achievement. The requirements for this achievement aren't clearly stated; in reality, you just need to perfectly parry the Prisoner's seven-hit spinning slash. Keep an eye out for it, and you'll be able to complete it.
How to Unlock Peter's Perfect Parry Achievement in Mortal Shell 2
After being executed by a basic attack for the first time, the Nameless Prisoner boss will use a spinning attack that swings seven times with his entire body rotating. Note that this is not the move he uses right after getting up.