This is similar to the Devil By The Well quest but much harder. Make good use of the Yrden trap and you'll have Jenny beaten in no time.
You'll find this contract on the noticeboard in Midcopse. Go and see the villager who will tell you Jenny wants to see you. Talk to Agneta and then search the fields beside the village using your Witcher senses.
In the fields you'll find torn fabric and footprints. Follow the footprints to find a knife, which you should take, then follow the trail of the dragged body.
At the shallow grave read the letter and then head back to the village to talk with Ealdorman Bolko.
You'll learn that you're hunting a Nightwraith. Get over to the lovers' meeting spot and meditate until the sun goes down. Jenny is vulnerable to Moon Dust bombs, Specter Oil and Yrden, the magic trap.
It's the Yrden trap that's essential here. Unless Jenny is within the circle of the trap, she won't take any significant damage. Cast the trap and step outside of it so that Jenny will step into the circle. When she's there hit her hard and fast.
She'll disappear at least once and three other Nightwraiths will appear. These don't need to be trapped and can be killed with one blow, so they're a nuisance rather than dangerous. Once Jenny reappears repeat the Yrden trap trick until she's defeated.
You'll receive 240 EXP for your troubles and the contract is complete