Did she solidify her magic? I'm just guessing but it might be that for them, magic is like a liquid, hence those circles that we can see whenever they use magic, but Serina changed it from that "liquid" state to a "solid" state which turned the magic particles(?) into a cube, making them solidify and be able to momentarily block the attack. If two people are fighting with their magic being a liquid, I'm guessing it's like two waves of water going against each other, the more water you have the more likely it is to push back the opposing wave, but what Serina did was basically create a dam that completely stopped the water.
I might be just completely overthinking it and in the next chapter all this will be wrong, which would be really embarrassing but oh well, I'll just throw my theory out here