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.

She sticks with the bridge even after she's got it anchored to the bank, to manage it if they set it on fire again and shore up anything they hack to pieces, but when she's not trying to build a bridge at the same time as everything else she can run around much faster. Handholds can break off the bridge as suddenly as they appear once they've done their stabbity jobs. Her sword can spend some time with her full attention.

Then she can have a front row seat to the yet more frantic efforts to dislodge the beachhead before the reinforcements arrive. It's perhaps an exaggeration to say she's irreplaceable here, but she's certainly very helpful; her rendering the bridge extremely difficult to climb allows them to almost entirely ignore one angle of approach, and once she has the time to give it her focus her sword proves its worth very quickly. Whenever she flares it, it goes from driving back the darkness to outright overcoming it, and while the demons can replace it when she stops it's time their most dangerous combatants aren't spending fighting. The added sturdiness is a comparatively smaller factor, but with the intensity of the fighting currently ongoing it almost certainly proves decisive for someone.

 For a few moments it looks like the demons' efforts might eventually prove successful regardless, but then cold iron lances start speeding into the melee and the abyssal advance turns into a retreat. It probably wouldn't make much to turn that into a rout, but even as things stand they're almost certainly going to be able to run down most of the escaping Dretches.

Why not make it a rout? She's not done gloriously stabbing demons yet. Some of her people probably still have third cousins they haven't told about how cool she is.

The faster groundbound demons are about as fast as the fastest humans, which is to say that they absolutely cannot escape either Kybele or the cavalry that rides with her in the short run. They can and do scatter to make her job harder, but in a sense that's also a win; a demon that goes fleeing off into the waste at top speed is unlikely to reconvene with its fellows any time soon even if it does escape, and it's not like she could fully abandon the bridge to haul after them anyway. The rest of them do their best to make tracks, prioritizing terrain the horses will have a hard time following, and abandon their slower or less lucky brethren to the crusaders.

The flying demons get away just fine, of course, but despite their individual power they were never anywhere near the majority of the forces involved.

The army continues to stream across the bridge, but by now they're mostly not needed for direct combat and can set about securing the position and collecting bodies. The living ones go to a hastily pitched medical tent for channels as triage permits, while the corpses are laid out for their fellows to identify. They're still close enough to Mendev that it's possible to send them back in the empty wagons to be buried, outside the reach of petty cultist necromancers, and some of the veterans of the fourth crusade are determined to make the most of that while it lasts.

Can she find Irabeth not-busy for thirty seconds to ask her what the etiquette is about the dead, does she need to say a few words or anything?

"If you mean the funerals, the priests will handle their last rites, but I don't think it would be impossible to attend or anything. If you mean speaking to the troops... today went well enough I wouldn't say it's necessary, but I doubt some reassurance and encouragement would go astray."

"Anything more specific than that? People - think about death extremely differently where I'm from."