How to beat the Katakan?

I think regardless of difficulty, whether Death March or Blood and Broken Bones or easier, a lot of people have trouble with this fight in Now or Never.

Like the werewolf's regeneration in the Wild at Heart quest, if you truly can't manage to sustain enough damage (such as taking advantage of stun locking and staying very close to the enemy rather than casting signs), you'll want to try another approach.

However, do note that Thunderbolt + dodging is still practical, though it will certainly be a challenge if you want to keep trying! One thing that you can try in addition to make it easier is to cast Quen after several attacks (when it attacks), which will effectively negate the Katakan's attack allowing you to continue your attacks (though depending on your stamina regeneration you may still need to do some dodging). This works with other signs like Axii to stun lock, which should allow you to continue your barrage of attacks.

A more tactical approach probably is going to use status effects, which apply a damage-over-time effect that will DISABLE its regeneration for the duration of the effect. These effects can be anything from Bleed, Burn, and Poison. So if you have any weapons (or crossbow bolts) with these effects, it would be appropriate for this fight. If you don't, there are some other options you can try instead or in addition:

• Black Blood potion: The same one showcased in the A Night to Remember CG launch trailer for Witcher 3, this (and the Enhanced and Superior versions, which will be more effective in this fight) can be used to apply a bleed effect when you are damaged by the vampire as well as other necrophages. This can be purchased in Novigrad.
• Bombs like Dancing Star or Devil's Puffball: Bombs which apply effects, burn and poison respectively, can keep a fight in your control. You can also probably pretty easily upgrade them if you already have the upgrade formulae. If you're missing the base bombs, I believe you can find each formula at the Herbalist Hut near Oxenfort.
• Igni: Though you mentioned trying this already, using Igni to apply a burn effect will also work, though ideally you'll want to boost sign intensity (in general or with runes or visiting an Igni Place of Power) and/or use the Level 5 upgrades for Igni which boost burn chance.

Other than that, remember that you can stack up other advantages if you're still having a hard time or simply want to be prepared. Using Vampire Oil or crafting/looting a better silver sword if you haven't already will be effective here, of course, just to get the extra edge.

Best of luck in your fight!

I meditated until noon before I went in. It died in like 15 seconds. So, if you're facing it at night, just wait til daylight.

Make sure you drink a golden oriel potion before the fight. It makes you immune to poison and the Katakan's health regeneration is a result of you being infected with his venom. After that keep hitting him with igni and a few silver sword attacks and he'll go down in seconds.

I had the Axii sign leveled up. Once you use it to ,have him on your "side" he'll just stand there and you can hit him for a longer period using thunderbolt will be faster.

I had troubles with this one as well... Took me a while to figure out a method to kill him, but once I did I pretty much annihilated him.

Here's what I did:

1. Used Quen and switched to Axii before I ran in
2. Once I ran in I took a Tawny Owl potion and a Thunderbolt potion.
3. Kept Spamming Axii as soon as my stamina regenerated and spammed heavy attack
4. Killed before my pots even wore off!! Woot!

Moon Dust bomb disables the regen, so bring those. The effect isn't permanent, so bring thunderbolt to increase your damage. The Katakan really doesn't have much health when his regen is disabled.

I was having trouble killing it so I read here and meditated till afternoon and then went in with potion applied to my silver sword hybrid kill and then used fire attacks and fast attacks it died quick :)

What I did was waited till around noon and before I went in I used thunderbolt and Tammy owl and he only lasted about 15 secs.

I was able to defeat him recently with a rather cheesy quen and nonstop fast attacks when he was backed into a corner.

It takes a while to get him in a corner but once there he never gets a chance to hit back.

Just got to this level. Died a few times. Am currently using tawny owl potion thunderbolt potion and necrophage oil on my silver sword. Just got him down to about 1/4 health and he started barraging me with hits. Lol

20 minutes later. Continued to try using tthe above combination. That oil and potion combo. Mixed with the axii sign to keep him from hitting you as much worked.

Took me many times but I drank a thunderbolt before I entered the room and straight rushed it with mainly power attacks and igni. Got lucky and killed the bastard lol I'm level 16 though