Instructions
Open the prayer book of your priest or druid. Memorize the spell Harm. This lowers the enemy's magic resistance to 1. To be safe, memorize the spell multiple times.
Go to your mage's spell book and memorize an arcane attack spell. Magic Missile is particularly effective, and even low-level mages know it. Again, memorize it multiple times to be on the safe side.
Rest so that your spellcasters can cast their new spells.
Find the werewolf. Immediately cast Harm on it, followed by Magic Missile. This will kill most werewolves in a single blow. If necessary, cast additional Magic Missiles until the werewolf is dead.