Guys,
If you're on
Steam - you should not experience this issue anymore if your game is fully updated.
If you're on
GOG :
Go to your GOG Galaxy client and go to MORE -> Configure, then in AUTO-UPDATING section select 'DISABLED', then uncheck broken DLC. Once the DLC is uninstalled, exit GOG Galaxy completely.
Navigate to
www.gog.com, log in there and go to Account -> Library -> The Witcher 3 and select DLC 'New quest: Skellige's Most Wanted' from DLC tab. Download the offline installer and install your DLC.
Once the DLC is installed again, start The Witcher 3, load a gamesave prior to the fight with Werewolf and try defeating it once again.