Compare the small, temporary shields that Chris is making while retreating with the huge dome that Kat is creating and holding up there, and the difference in power should be obvious.

So, what can you, as an attacker, even do to get through something like that?

One possibility is applying as much force as possible onto a single point and try to pierce through the barrier, tearing a hole in the spell texture and make it collapse.

The other possibility is to apply some big chunks of force over a big area of the shield, weakening it considerably in several places, so that it’s imossible for Kat to sustain it everywhere and make it collapse that way.

For the second one you’d need something that creates a sudden burst of energy. Like an explosion. Ideally several of them at once.

… oh crap.

↓ Transcript
Shara: <whispers to Gideon> I can't GET to the sword! It's LOCKED UP! KAT has the key!
Gideon: Then GET IT FROM HER!
Shara: FUCK!
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<Meanwhile, Eddis shouts after Chris and Fernanda, who are retreating>
Eddis: Just GIVE UP and make this easier for all of us, Chris!
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Chris: SHUT YOUR DAMN MOUTH, EDDIS!
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<Katharina and the other civillians are trying to look out of her shield bubble>
Katharina: W-what is happening? Why aren't our guards fighting back?!
Prof. Brahms: Oh no ... they're coming ... they're COMING!
Katharina: I-I think I can keep them out for a while, but ...
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<The thugs take out some alchemical bomb cocktails and light them>
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<They throw the bombs>
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<The bombs fly in a wide arc over Chris and Fernanda and are heading to impact on the shield>
Katharina: ... oh CRAP!