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!
