The skin of the Giant Worm was really hard to penetrate or pierce with blades, many fire mages could probably make some damage with lots of time. What they didn't have, fortunately, ever armor has a weakness and the one of the Giant Worm was the inside.
Gremory spent half his mana positioning Dark Bombs on Carl, he could use a higher amount if he was placing in something more durable than rocks. But that would also risk the life of Carl, fortunately, the amount used was enough to kill the beast from the inside.
Now, the Giant Worm is just a dead creature, filled with darkness, even more than what Gremory found on Geoffrey's body. Much more, and it was calling for Gremory.
