Alt-text: "Eddis is having way too much fun with this."
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.