Does it worth lvling witchcraft for Soulsap? Coz enemies often have high bodybuilding so they can easely resist it.
The skill Divine Light from the Man-At-Arms tree lowers the target's Bodybuilding and Willpower. Maybe there are others, I am not that familiar with all of the skills. edit: The wiki says Soulsap is resisted by Willpower, not Bodybuilding. In addition to Divine Light, you could use the Witchcraft skill Drain Willpower then.
