Volcanoes were dangerous not only because they could spew lava flows at any time, but also for their instability.
Sure, a small tremor or even a bomb blast wouldn't be enough to make a huge volcano erupt lava, only to shake the top layer of lava, nothing more.
But... six powerful attacks from K1-Phantoms capable of fighting against Top-20 monsters could bring significant destruction.
Adam, Katrin, Oskar, and the others knew exactly what would happen after they unleashed their attacks deep into the volcano, but they had no other choice.
It was far better to risk getting burned by lava than to fight an evolved Prime Thunder Demon.
It's worth noting that since it was lava in Black Cinder Forest, it was actually dangerous to both monsters and Phantoms.