If you've chosen to battle the frightener, you'll get the energy regeneration potion from Vesemir. It will come in handy during the following battle, as you'll be forced to use Aard a few times.
First help your mates to get rid of the bandits. They will all engage the frightener in a while. Unfortunately they won't be able to hurt it unless you help them out.
The frightener is vulnerable to sound. Make sure that all the cauldrons and the main bell make some noise at all times. You can make the bell sound by touching it, and the cauldrons require the use of Aard. When the bell and both cauldrons make noise, grab your sword and help your friends defeat the monster - your blows are stronger to theirs. You'll probably have to move the bell and the cauldrons a few times before it's over.
When the frightener is defeated, Triss will appear on the courtyard. You won't be able to help her for now.