I've seen a lot of posts complaining about Viper / expressing disbelieve at how hard the boss is, which I totally get. Until you get reflect, that is.

It feels like this boss is meant as a tutorial to teach you to use the reflect skill, it changes this boss from a challenging slog to a super fun massacre where the boss barely gets to move more than twice before being stunned. I went from barely getting to the 2nd health bar to beating him 3 times in a row with plenty of heals left once I started using reflect.ö

Grab reflect in the common tree, get the upgrades (most importantly the speed one, but the other's can't hurt). Then reflect his slow moves. I think you can reflect almost every move except his grab / stab thing but I'm kind ass at parries so the ones I parried were:

Phase 1

3rd hit on his stab combo. (Perfect) block the first two stabs, reflect the 3rd delayed hit.

2nd hit on his delayed spinny combo. (Perfect) block his first hit, reflect the 2nd delayed hit.

Reflect the upwards swing he does before jumping back (usually followed by a weapon throw). It's very telegraphed and easy to reflect. If you're unsure on which move he's throwing out just block, you'll have enough time to reflect once you realize that it's this move.

Dodge the grab.

(Perfect) block everything else. I'm sure there's way more stuff to reflect but I'm not skillful enough / didn't bother learning any of the other timings.

Phase 2

2nd phase is more of the same. (Perfect) block his lunges (maybe you can reflect those? I didn't bother to try), reflect the same ones as before. Just hit him hard with a weapon skill / heavy attack whenever you have the time.

Phase 3 (2nd health bar)

This phase is actually quite a bit easier, even if it seems super intimidating, as he loses his projectile moves (except for a stone toss that's super slow) and his grab.

Reflect the fist slam (first move he does). It's so telegraphed you have more than enough time.

Reflect the upwards ground clawing attack. Similarly telegraphed. If you don't feel comfortable reflecting this, just block it, he usually does the slam afterwards.

Counterattack the move that puts the P sign on you. Fun fact: Even if you instinctively reflect (which I did a couple times), you still have enough time to get the counter attack out.

(Perfect) block everything else. He leaves you ample opportunity to attack with some weapon skills in between too.

Phase 4

Just (perfect) block everything other than the upwards clawing attack and fist slam. You can prolly reflect more stuff but it's not necessary. He's restricted to very few moves and they're all super telegraphed.

Edit: I gave it a try at it seems like you can just reflect the first dash in phase 4 on reaction when you hear the scream, which makes that phase insane DPS for you. Might depend on distance though.

That's it, the viper is now your bitch.