Note: This solutions contains some spoilers.
This achievement is tied to the level 22 Witcher contract with the name 'Mystery of the byway murders'.
You can acquire this witcher contract by reading the notice board notes that you will find in the town of 'Oreton'.
Once you have picked up the notice you will be directed to talk with the person who has put up the contract. He will send you to a village that has been overrun by Ghouls.
When you arrive at this village you will notice that these ghouls are all level 20 and are somewhat less easy to beat than regular ghouls. Especially since ghouls are able to regenerate health. I personally used my 'Puppet Axii power'on them and then made sure i slashed them away from the group one by one but all in all the same strategy applies to them as the one that applies to fighting regular ghouls. Hit fast, dodge away and hit fast again. Just don't get stuck between them.
Once you have killed the ghouls then the villagers will come out and it more or less occurs to Gerralt that something feels slightly off. Upon inspecting the village with your witcher senses you will notice that a stench trail leads you to a locked door. The key to this door will be handed to you by the villagers after a brief conversation.
Important note by 'A Silent Circus': ( thank you for pointing this out)
I just wanted to add that this achievement CAN be missed with the wrong dialogue choices. If you don't question the villagers when they're acting shady then they won't give you the key and you can still turn the quest in but MISS the achieve.
Inside the hut that you have just entered with the key you will find a trapdoor in the floor and beneath it is a cave.
This cave contains Sarasti at the lowest level. You might wanna ensure that your potions are stocked up and if you have the possibility to do so ,apply vampire oil on your sword. You are about to face a rather nasty vampire enemy.
He is weak against :
-IGNI ( The fire sign)
-The Devil's Puffball grenade
-Vampire oil.
Truth be told; the strategy i used on Death March was to simply cast a puppet spell ( the 2nd spell you unlock in the Axii tree) on him and punch his teeth in with normal sword attacks. If executed properly you can just take him down while he is not able to attack, nor defend himself.
After that is done you can go and collect your reward.