[How did he…?]
Czac had spoken like he was aware of the danger of blocking the Barrier Bullets, with magic barriers. Something that he was definitely not supposed to know considering he did not know of Dominik's existence until a few weeks ago.
The Barrier Bullet rare skill was one that had its damage multiplied when its target attempted to block with any kind of energy barrier.
The magic spheres created by the skill absorbed the energy used to make any barriers that were used to block it upon contact.
Depending on how much energy was used to make the barrier, the damage of the barrier bullets would increase accordingly. The stronger the barrier deployed, the more powerful the barrier bullets would become after absorbing the energy used to make it up.