I've managed to beat him once, in the Scoia'tael path, but I decided to go back and try again in Roche's path. I noticed that every light hit geralt does, he deals minimal damage and heavy ones deal only 10-15. And I'm using the Robust Cutlass with a Wet Stone in it.
Guess it's just another trick of the Dark mode.
Even though, the strategy was Quen, Aard, three hits and dogde away from him. To increase damage I used all of my throwing knives and bombs. It takes a lot of time and you only have to take his health to a little less than half the bar, so they'll speak all the lines and the finishing cutscene will come up.
Very tough fight.