Nithral Boss walkthrough during the mission Wandering In The Dark for The Witcher 3 on the hardest difficulty, Death March!
I cut the first two stages of this fight for time (because all three stages are essentially the same but the third is the longest). Use Quen to give yourself an extra shield, and Axii to stun. Use Keira as a distraction, but she will not do much damage, so use one or two heavy attacks after each stun then roll away.
Lather, rinse, repeat. Always repeat.