I ask this because at the bottom of the screen it shows her magic amulet.
I have no idea how to equip or use it or if it is of any use in this fight.

The magic amulet would be useless against Caranthir: he moves around and keeps teleporting. You just need to keep spamming Blink (Q).

Thank you Hickory.

You're welcome. Just remember that the amulet was given to her by Triss for personal defense: if she was overwhelmed by numbers, for example, Triss would rain fire on Ciri's position. It's not an "attack that enemy over there" spell.