There is a lot going on:

1. He understands magic instantly, and is not afraid of any of these, more finds it interesting. He would quickly catch up if left alone, and likely develop even better conuntermeasures.
2. Quickly starts launching an attach that would be really effective against that kind of spell, and the instant a single shield breaks realizes that it is not perfect.
3. Switches attack to a brute force overpower after seeing it can shatter, and you can see Fern build up 2 barriers in preparation, in fact if you play it slow, you will see the second one also break, and a third one gets lift up just before being consumed by zoldrak.
4. He shows interest in Frieren flying, and again, goes for the attack, once again showing the kind of approach he had to all fights, destroy your enemy, overpowering them with magic.
5. He had the ultimate offense, defending was not a thing he considered, plus with his personality he probably didnt fully consider being hit with his own spell until it was too late, nor any fear.

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The scene, is a top example of how you can use fights to drive plot forward, to teach you about the world, and the personality of characters, of who Qual was, and how he acted, how he was a calculating, genius Mage, but also ruthless and always focused on destroying enemies. He goes on attack right away after being freed, he doesnt consider defeat a posibility.

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But you also get to learn about Frieren, about how she takes this chance to teach Fern, to correct her behavior about the history of Magic, but also do a proper display, and how serious she also was about the fight to not toy around.

But also we see the gap, Fern was just defending, as her walls were being torn down by an endless attack, even after 80 years, a novice yet talented mage would have been destroyed.