Werewolfs are weak against Moon Dust, Devil's Puffball, Cursed Oil and Igni.
Werewolves are creatures with both men and wolves inside them. When in beastly form, they take the worst traits from each: the wolf's drive o kill and hunger for raw flesh and the man's cruel and calculating intelligence. A werewolf's condition comes about through a curse, and the transformations happen outside his conscious control. When he reverts to human form, he has no memory of his deeds — otherwise he would surely go mad and take his own life.
How to beat a Werwolf
One of the trickier enemy types in the game, werewolves are dangerous beasts that can outlast you if you're not careful. They're both agile and strong, so watching their movements is key to overcoming them. If at any point they draw close, or appear to be charging on all fours, be sure to roll to a safe distance.
However, it's not strictly a werewolf's claws that are the problem. Indeed, the biggest obstacle when fighting werewolves is their ability to regenerate their health at an alarming rate, almost whenever they feel like it. This ability can lead to some fights lasting minutes at a time, all while they're taking chunks of your health away with each blow.
To counteract this, try using igni to temporarily stun the monster, and potentially set fire to its fur. When it's incapable of jumping away or fighting back, lay into it with as many fast attacks as you can, making sure not to leave it on even a slither of health, lest it heal its wounds soon after.
The most effective strategy, however, is to make use of a moon dust bomb. These particular explosives will stop werewolves from regenerating their lost health for a time, making the fight far easier, assuming that you're able to bring the beast down in a timely manner.