I posted awhile ago about my trouble with the game's combat. I've since bought all of the inexpensive early weapons and have been playing around with movesets trying to find a combination of speed and range. I figure if damage can be upgraded, fast attacks and hitstun will primarily win fights.
I'm refuting my idea that enemy attacks can be interrupted leading to normal hitstun. If you R1 an enemy in a neutral state, it will enter hitstun. Some heavy attacks like Hunter's Axe transformed R2 or leaping R2 will always stun whether the enemy has started an attack or not. What was confusing me were all the times I swung into an enemy attack and it was stunned, yet other times it seemed to armor or poise through my strike and continue to hit me.
I believe enemies can be stunned with melee attacks the way guns will stagger them for a Visceral attack. I spent plenty of time fighting those Jason Vorhees lookalikes in Cathedral Ward. I started using Saw Cleaver's longer trick for as its R1 swings like lightning. If the enemy was neutral, R1 always stunned (they have to recover but remain standing). If I continued swinging and they started a slow attack (such as raising their right hand and charging before swinging), I could stun them again but it always seemed random. If I waited and timed an R1 once their attack started, it could stun. If I kept spamming R1 after hitting them initially and they managed to start a slow attack, sometimes my second attack would stun, other times it was my third. I could get no conclusive results but I knew it was possible under some circumstance. This lead to my second freak combat occurrence.
I abandoned Saw Cleaver and switched back to Hunter's Axe and fought Vicar. During that fight, it is possible to inflict enough damage that he will sit on his hind legs and howl, spurting blood and leaving himself open to a few free attacks. Cleric Beast does the same thing - it will howl, then swipe in a full circle around his body. This seemed to be from total damage alone, not any timed attack. I could score these sort of stuns on Vicar with any R1, R2, or leaping R2 - it always seemed to be from total damage (perhaps related even to his lifebar percentage, not combo damage). Anyway, I always used Hunter Axe transformed against him for range and damage, no guns. Yet last night I got a Visceral attack on him... without guns. The sound cue played and he fell to his knees, and R1 triggered the zoom-in cutscene that plays with every Visceral Attack.
Witnessing now that against a boss it was possible to trigger such attacks without guns leads me to believe stunning or full staggering enemies is related generally to timing with some exceptions. Against those Jason enemies like I said, R1 would stun except in some cases after they had started an attack. Full charge R2s knock them down unless they see you; they will swing right through your attack and knock you down. Leaping R2 always stunned as did standing R2. I could leaping R2 and mash stand R2 and I would interrupt every attack they started.
My Hunter Axe is +4 at the moment but my hope is all of this data is related to moveset and in certain cases attack timing, not weapon or stat level. My goal was to look for consistency in the combat system and I believe I found it. While the design is not what I hoped for, at least there are ways to generate setups and guaranteed damage against enemies, and this testing explained certain things like why standing R1s would sometimes stun but not always, or why full charge R2s would fail to knock down enemies. I played Dark Souls with this mentality, using particular attacks to setup potential for further damage. Just like one would learn to time and space dodging against enemy attacks, I feel it is important to learn how and when to attack after making attacks whiff, dodging attacks, or landing one attack and lead to another. There are enemies who can be pseudo-stunlocked with attack spamming but they are exceptions.
Knowing how enemies will sometimes fast attack out of hitstun or are able to basically poise through attacks due to impact timing discourages players from fighting in such a way. There are combos of sort to find, probably against all enemies, but they will depend on weapons and attacks rather than a number. This makes the system different from Dark Souls' Poise counter or the sort of stagger point count in Devil May Cry 3 (or other action games), but I feel satisfied that it is not in fact random. Hope you find this interesting, hope I can duplicate melee boss Viscerals, hope we can share more PvE strats. Onward into the night...