Rayne pulled out his second dagger, and ran at the beast, which was not focused on the invisible teen running at him with 2 invisible knives. He jumped onto the back of the creature, making sure his timing would prevent someone from throwing something at him, or shooting him, and cut the beasts other arm off with Carnwennan.
He stuck his other dagger in the creatures neck, and pushed himself off the beast, watching as it began to flail in what were its final moments.

The Spriggan was flailing about, unable to do anything without its arms. It kept screeching, even when it fell to the ground from a lack of balance.