How can I kill Jenny o' the Woods?

This fight is quite challenging if you are not prepared. After some experimenting I found the best tactic is to simply not get hit, this wraith deals too much damage in a single hit.

Before the fight drink a Wraith Decoction if you have one (it provides a Quen shield if a single blow deals more than 1/3 of your health in damage). If you have Spectre Oil apply it as well and use ThunderBolt if you have any.

Now that the fight has begun you will want to use the Yrden Sign and stand on the edge of the circle opposite to the wraith. This lures it inside the circle where it seems to become physical (the ghastly swirls around it disappear) and takes significantly more damage. Take this moment to get one or two strikes in with your silver sword but be careful to dodge (alt + direction) or roll (space + direction) out of the way of Jenny's swipe or this fight will end quickly. Repeat this, cast Yrden, lure the Wraith, land 2 hits and dodge.

At one point or another Jenny will disappear and three other wraiths appear. They are easily dispatched with one hit and you should do so as fast as possible (Jenny heals while this is happening and these wraiths can also drain you of health). As a last note, you should try to keep the fight close to the camp fire. I found that if I strayed too far I would end up with the trio of wraiths spawning in bushes or with trees and bushes between me and them. This added quite a bit of time to the total needed to kill all three, allowing more health regeneration of the target.

Killed her at level 6.

When the fight starts, cast Yrden. Don't be aggressive, she will punish you. Always dodge and counter attack. Take your time and don't let Yrden run out, and don't be caught casting Yrden when she attacks.

When she splits, run to the center of the 3 and fire your crossbow the moment the first one appears, twist to the next and fire, twist and fire, drop another Yrden and start again.

Obviously you want to use the spectre oil. I recommend that instead of juggling Quen and Yrden, you focus on Yrden and just keep swallow handy.

Don't get hit.

Here's my method. I play sign heavy and for whatever reason didn't have the proper bomb diagrams.

1) Hit Quen and keep it up whenever it blows out.

2) Lay Yrden traps

3) Hit her like 3-4 times and dodge or until she hits and kills the Quen

4) Run away from the 3 health suckers when they spawn

While Yrden keeps her corporeal (as Geralt hints by saying something about keeping her bound to one place), Quen keeps you from getting gutted, and the health sucking wraiths disappear after only a few seconds... rendering them completely worthless to her if you run a few feet away. If you stick around to attack them, they have time to regenerate her health. Enabling "sustained glyphs" helped. I didn't need to use any materials doing it this way.

EDIT since I can't comment on others' yet...

Reading other people's methods makes it clear to me how variable this game is and just how differently a single fight can be approached. My Quen/Yrden approach probably only worked well for me due to the fact that I have both fully developed with 2 mutagens and additional bonuses on my gear boosting the intensity well over the 30% mark. I only needed to use each about 4 times for the whole fight. Most monsters can hit me 2-3 times before a single charge blows out, and I can lay multiple traps that last minutes, so she never gave me damage.

I feel like keeping Quen up in this fight doesn't help. fiddling around moving between Quen and Yrden makes you more susceptible to being hit and it slows the already incredibly slow fight to a halt.

My recommendation is do the whole fight without getting hit and don't cast Quen once. Also make sure to maximize hits inside Yrden by luring attacks right at the edge of the circle. If you notice you will fall out of the Yrden circle on 3rd/4th hit its important to dodge back instead since she loses all hit-stun when she falls out of the circle and can instantly hit you back.

Read the bestiary. It's written that she (as any wraith) is vulnerable to Yrden, Specter oil and Moon dust. The latter bomb materialises the wraith for some time. I found it much easier to kill it than with Yrden, because the area of the sign is pretty small, and every time I attack, it made the wraith out of the circle and hard to beat again.