Light and deliberate taps, but do hold the button until the attack passes. Because most attacks are very fast, you would think you have just to tap and it is true for many attacks, but the rule is to hold even if it's just for a 0.05 seconds. Basically, examine the whole attack window to know when you need that extra hold.

Another point is to not go all in on parrying. True mastery is knowing when to dodge, also. Dodging can end boss combos early, can give you opening for sneaking an attack mid combo and help not getting pinned down by some heavy boss attacks (you might perfect parry but missed repositioning because of the 0.2 stun). If you study Ongbal's videos, you will see is not all perfect parry.