Yaerius: btw. I had a horrid experience with this quest. the last battle is just so damn hard (or maybe I was a noob in fights at that time - will see on second playthrough) hardest fight I encountered ever. keyran is weak comparing to this.
PS. I play on hard. maybe it's nothing in lower difficulty levels.
Re-spawning three on one wraiths and multiple directions you have to cover definitely make this fight tough without a bit of preparation. With the right tactics it's a breeze though. Come equipped with a fair number of traps, some Red Haze bombs, and Specter Oil and the whole thing is suddenly a lot more manageable.
Lay your traps out in a ring surrounding the area where the witch is working for some easy early damage, then stay on the move to draw the wraiths into a tight group. When they get close enough to one another toss a Red Haze in the middle and you will -usually- cause two of the wraiths to go berzerk and attack the third, flipping the odds from 3 to 1 against you to 3 to 1 in your favor. Once that happens I'd just stay out of it and let them tear one another up, there's no point in getting in close and accidentally breaking the effect by hitting one of the wraiths on your side. If you get a bit unlucky and your Red Haze only affects one wraith, you can either fight whichever wraith comes after you one on one or use Axii to forcefully convert it to your side. If you choose to fight it I'd suggest having Quen up. Wraiths do absolutely brutal damage from the rear, and you don't want to get 3/4 of your vitality knocked off just because you were too distracted to notice a wraith respawning somewhere off screen.