Should you find yourself facing the werewolf, the spell to kill should be performed as follows.
Agronium Agnotum
Remember: The werewolf will show you no mercy. Kill it if you can, if you don't, it will kill you.
Dolores J. Umbridge
The Shrieking Shack //
Snape's Dungeon //
The Gallery //
The Library
ag-ROW-nee-um ag-no-tum
perform a swift flick and aim your wand tip at the werewolf's head or chest for an instant kill. A strike on any other body part will injure it, thus enraging it.
Sen. Undersecretary to the Minister
The Library Annex //
The Theatre //
What's New
The Floo //
The Kitchen //
The Marauder's Map