Guida contratto di Sarasti in The Witcher 3 Wild Hunt [PS4 - Xbox One - PC]
The solution for this particular side mission of the game.
Trophy Guide / "Vampire Slayer" achievement
Sarasti's mission starts from the Oreton mission bulletin board, a village west of Velen. From the bulletin board you will need to take the note "Contract: Missing Soldiers".
Talk to the village commander and then head to Byways further south. Here begins the most difficult part of the mission where you have to kill 5 ghouls. After sending them to the ghoul creator talk to the villagers and follow some clues to reach the catacombs below the village. Here you will have to fight Sarasti, a vampire boss.
This boss doesn't have much health so the fight shouldn't be particularly difficult as long as you have a character who is at least level 20. With some quick attacks you will make Sarasti falter and stop him from attacking.
Talk to the villagers after killing the monster to unlock the achievement and get the reward.
Here is a video showing how to complete this contract, good luck!