Well obviously it's a problem if someone sets it on fire.

Right, so focus on any Brimoraks and have an aqueous orb or two handy, they can do that.

Two days later, the army arrives back at the Sellen for the first time since Kenabres. Their approach must not have been perfectly subtle, because there are some demons clearly in position on the other side, but not as many as they had been expecting at the ford. The archers deploy themselves up against the river, their arrows ready to provide fire support, but they make sure to spread out enough that they're not begging for a fireball.

The second bridge comes together faster than the first because it's more execution than experimentation, but it will still take a while.

Fireball.

A number of the demons present are Brimorak, and their flame magic is both extremely long range and very useful against most structures even if they aren't literally paper.

They can prevent half of those, but three of the beads get through to slam into the bridge and detonate an an explosion of flame. It's not enough to do more than lightly singe Kybele through a resist fire, and the spell seems to work just fine for protecting her gear, but several areas in the bridge are now burning merrily.

She can smother those spots with more paper, but she has to concentrate on them one at a time.

If ‘wet’ is less distracting than ‘burning,’ then the orb of water that starts working on the parts farthest from her is probably at least a little helpful. Their archers are a bit more proactive about things, with several volleys loosed into the area the fireballs are coming from, but the range is long enough and the cover good enough it only produces a few corpses.

Wet paper is paper and ash is not paper so the orb of water is very helpful!

Once the bridge is not actively on fire she can make more forward progress.

There are two more fireballs, and a few demonic archers willing to try their best to shoot her, but none of the demons here are especially powerful, so if they want to hit her they'll need to wait for her to get closer or vice versa.

Which is why there are over a dozen invisible quasits bearing down on her position.

About that.

It's a funny story, but there's only a handful of demons here who can see invisible people and they've just thoroughly demonstrated that they can't really hit someone from this far away. Whereas the crusaders have a bunch of archers and scary paladins who can detect evil demons. So what if they just... didn't? Most of the Quasits break off the approach, either curving low along the water to go after some less dangerous humans or just outright leaving. A few of them continue onward anyway, but when they realize their fellows have ditched them they decide discretion is the better part of valor.

Instead it's a bit less than two hundred feet from the far side when the next attack comes, this time from gaunt, red skinned demons lurking behind rocks. Whatever strange magic makes up her bridge, can it be dispelled? And if just one attempt doesn't work, does three or five or twenty?

That doesn't work at all and still doesn't after more tries! It's paper! It's real!

Alright, the next plan is to wait for her to finish the bridge, and then as soon as the soldiers start to reach the other side, drown them in noxious gas as our front line demons start hitting their vanguard-

Noxious gas? Got it, they can do that. Oh look, she's in range now!

All of her surroundings are suddenly filled with a bright yellow haze, and it smells absolutely disgusting - like sulfur mixed with bile and a hint of pure, nauseating evil.

...giant paper fans??

That'll quickly bring it from nauseating down to merely disgusting, and then about half a minute later disperse it entirely. 

Great. Does she need to be at the end of the bridge to keep moving it for the last stretch? They've got a few guys with spears and heavy shields they'd like to put between her and any charging demons, but if that's impossible they'll settle for supplying her a safe place to retreat back to.

Her range is not amazing but she doesn't have to be at the very edge, she can hang back enough to let a few guys stand in front of her.

They'll make the most of it.

A few moments before the paper actually touches the ground, a trio of Babau attempt to push their way through the shield line while their goatlike Schir counterparts attempt to scale the side of the bridge and go straight for Kybele. The noxious gas also makes a return; it's a little less thick, but that's hard to spot visually because the front lines are covered by roiling darkness lit by flashes from glowing weapons.Tthe demons seem completely unbothered by either hazard, but some of the soldiers are struggling with it.

The bridge can have spikes on its sides! Very suddenly! The fans fan! She's got a sword now!

To the great surprise of the Schirs, the paper slices at their hooves and hands; the added foot- and handholds might make the climb easier, but they come out of it already bloodied into the face of more spears, which stymies them for a few crucial moments. The defensive line contracts, bowing under the weight of fending off attacks from all directions even as their senses revolt on them, but enough of them survive that the lines don't buckle before the fans can get rid of the gas and equalize the fight. Her own efforts play no small part in this; her sword glows like a beacon, driving back the dark wherever she goes, and it proves able to cut through weapons and flesh alike without the slightest concern for a Babau's acidic skin. Any attempts with paper meet a far more ignoble fate, but it only slows the efforts on the Babau and the other demons have no such defenses - the attack falters wherever her attention turns, and before long the surviving demons are pushed back far enough that the vanguard can step onto the rocky ground of the far side.

That's their cue! On the far side of the river, trumpets blast as archers step up their shooting and cavalry thunder across the bridge to join her.