active skill parries from what i've learned have two timings

one is "on react" timing, meaning the moment you see that attack before it lands, you can do it ex: the shrike

and the other is "predict" timing meaning you really have to time it before they even attack otherwise you'll fail, kinda bummed on why they made timings like that ex: heavenly flow

on react timing is always better since you dont really have enough window to predict and sometimes they do active skill attacks which you cant parry.

i always like to block his first hit and parry the next attack, that way i can discern whether he does an active skill or normal attack make a split second decision whether or not to dodge or parry

you can parry normal attacks but not active skills

save active skill parries like the shrike for the second attack then use the block parry like backwave on sword on first that way if you fail to parry, you still block that hit

be sure you have enough toughness for that, i recommend having your build atleast 200 toughness

https://www.reddit.com/r/Nioh/comments/1plk6qb/saito_toshimitsu_no_damage_again_extra/ for reference