I played this quests at level 7 on normal and it was still pretty hard so consider coming back later with a few more levels if you're playing on death march. Cursed oil is really helpful, moon dust bombs too if you have them. Tawny owl is a really good tip too because it's really hard to see you can't fight this monster at a certain time of day to gain an advantage because of the quest requirements. The quest forces you to wait until that certain time of day for the monster to appear.

I didn't know igni can stop the health regen! If so that's great but I remember using igni a lot so maybe it's only temporary or maybe it did the howl animation again and I missed it. I know it shouldn't be and that's my problem, haha! Right from the get go it's spamming heals and I can't damage it. Igni isn't stopping the regen for me, Cursed Oil is barely an improvement as it still regens far too quick and I don't have Moon Dust Bombs yet. Considering PainOfDemise defeated it at Level 5 using Igni I shouldn't have to go through getting Moon Dust Bombs though I wouldn't think. Unless taliesin2943's theory of time of day etc.

Witcher 3 wild at heart werewolf heals