Shoot it -

Nothing that a seventh circle wizard couldn't do if they got a bit lucky, not until the real prize is on the field - 

He does not fear for himself, at any point in this. The mist conceals him until almost the last instant; a dozen invisible summoned bodyguards cover him even then; three of the twisted mirror images go down to lucky shots, and all three archers buckle, whimpering in horror, when they hit. He wasn't expecting any of them to hit. Between the magic in his bracers There is very little on this or any battlefield that can threaten him; fully prepared he simply has too much magic protecting him for most of the archers to stand a chance at hitting him. 

The enemy's wizards and angels will probably try something. He has deafened himself, on reports of the trumpet archon. He has a contingency if incapacitated or controlled, to remove him from the battlefield. He has a greater spell immunity up at a fairly insurmountable caster level, covering Holy Smite and Trap The Soul and Disintegrate and Heal and Holy Word and Undeath to Death. Some of the beads around his neck are of a special poison that is contained by magic and so, in an antimagic field, will explode into a cloud of fog that'll kill or at least incapacitate any living creature in the antimagic field with him, living creatures' defenses against poison being generally magical in nature.

 And if you defeat all that and destroy him, so what? His phylactery is in a secure demiplane where magic doesn't work except for a minor artifact that lets him escape it. There are threats to a lich all the same, but not many of them. Imprisonment? Tar-Baphon would free him, and it'd be inconvenient mostly because he'd then owe Tar-Baphon a favor. Trap the Soul? Even when he doesn't have the greater spell immunity up, his name is long lost to history, and his spell resistance is very good, and among the many many magic items he wears are those that allow him, in an emergency, to burn them for advantage at a crucial moment.

So the thing with the best odds that a seventh circle wizard could do is - Plane Shift. Lucky seventh-circle wizard, guessing something he didn't give himself immunity to today. (It is more likely to fail than to succeed.)

He aids himself with his lucky talismans; he ignores the spell; he reads a scroll, Wail of the Banshee, and a horrible inhuman screech echoes out where it can be heard across the whole army, and every man within forty feet of General Alba in any direction dies on the spot, without exceptional fortune. We'll see how the army fights with no head.

No, they're all fine.  (Two men crumble to dust, just outside the range of the power with which she's protecting them.)

- how - 

Plane Shift.

 The monstrous beast vanishes from the sky.

But there are, of course, others, dozens of them, striking at the heads of the army's many regiments, at its notable heroes, at its few flying cavalry -

- And the army is emerging from the mist.

Some soldiers see it. They point and yell and fire arrows at the mist that, instead of burning off, drifts unnaturally closer. Some yell it's a trick or can't you see them, but those are the minority, and have an odd tendency to die.

Some of the soldiers of Tar-Baphon are orcs of Belkzen. Some are Ustalavic men-at-arms and knights, other are sellswords of every race and nation, working for the person who pays them. The River Confederation maintains its own armies on the eastern side of Lake Encarthan, of course.

But the troops emerging from the fog? There are orcs among them, but those are few. At the head are the dead. Zombies shambling forwards to soak up arrows, wearing the clothes they wore in life, skeletons still wearing their old armor. Ghouls charge forwards with the axes and shields of the infantry raised and their own fangs bared, bone centipedes of a hundred corpses stitched together lashing out with their bony fangs - and in the midst of this are their officers, bone priests with skullfly amulets and raised scythes, spawn of vampires warded somehow against the sun's rays, hard-eyed men with slit throats who never speak, the daughters of Urgathoa screaming and howling above it all -

And from the fog come more, and more, and more. They are not men. They are the ocean.

And then there are others. A great skeleton with a horse's head taller than a giant, not bones but a whirlwind of bones making up the skeletal frame, roars above the confusion, and men die by the hundreds. An entire warship of bone and muscle appears above the battlefield, and boarders swarm out to drag every soldier near them onboard as missiles shred the cavalry around it. An animate siege tower formed of bone and flesh rumbles across the battlefield to smash a battalion flat beneath it, drawing their broken corpses into it as it charges onwards. A flayed giant erupts from the shell of an officer to tear the mortals around it to pieces barehanded, even as a skeletal stag and its laughing elf-corpse rider call thunder from the sky to smash horsemen flat.

In all this chaos, a man dressed like any other man save that his cloak bends oddly turning and fleeing from the host, brushing against dozens of others who are suddenly stricken stock-still with terror, is nothing unusual at all.

They are not all going to die here because if they were, Aroden wouldn't have sent Arazni. 

....a lot of them are going to die here, admittedly. But die winning, not die losing. It matters. 

Smite Evil boat. Go fly up and stab evil boat.

The Second Army does not lie down and die.

Skeletons and zombies are arrow-resistant. Ghouls are less so. The light bows of the archers normally do very little against armored enemies, but they have the blessing of Arazni, who while she is a little bit a god is much more importantly someone who can cast Aroden's Magic Army without needing a miracle. They go through undead flesh and bone like it wasn't even there.

And the cavalrymen's horn bows, the bows of Taldor's knights, the finest in the world, now with Arazni's blessing? There are paladins in every group of them, keeping their spirits fresh and them inspired. Some of these paladins are very, very good.

The siege engine dies. It dies because it is shot sixty times with arrows within six seconds of emerging from the fog, by enemies who dart out into the gap between the armies and empty their quivers without ever slowing down. Since it is made of bone they use target arrows, which when blessed by Arazni turn it into just a very grotesque monument. The dead elf dies. She is a thousand feet up in the air and protected by a really astonishing number of spells and she still dies because Taldane cataphracts can shoot straight up with no particular problems and anyone still alive after the first decade of war is really astonishingly good at being alive. 

And as the massive host of undead surge into their ranks, the spearmen hold. They hold with skill and they hold with honor and they hold with the blessings of the gods, dispensed by clerics prepared to channel as needed. Large shields and lamellar armor and lots of combat experience mean that the ghouls and the skeletons and the zombies don't actually last very long. They are meat for the chopper, and then they aren't.

The undead don't stop coming out of the mist.

Holy Word, Holy Word, Holy Word, Holy Word, clear of the ranks of the soldiers because she'll kill a bunch of them too if she gets near them. Any astral deva could be doing that.  A few more attempted Plane Shifts aimed at terrible things that did not bother to make themselves immune to that spell because the Crusade didn't to their knowledge actually have any demiplanes from which they couldn't be promptly retrieved. Holy Word, Holy Word, Holy Word, Holy Word.

Iomedae has her sword Flaming, the better to fight flying boats with, and Defending just in case the boat hits back, and she savages the boat's side until it manages to swing around and aim its guns at her particularly, at which it blows a hole clear through her and sends her flying through the air and into the mists of the undead. Her instinctive Lay On Hands as she dies picks her up again and now her only problem is being deep behind enemy ranks, but she can fly -

Iomedae can do what she likes. The fundamental problem the living face isn't one of heroism. It's not even one of archmages. The Taldan army did not have time to fully deploy for battle. The Taldan military doctrine is to use the infantry as a fortress around which the cavalry wheels and maneuvers, striking with lance or bow at the most vulnerable points.

The Taldan cavalry cannot wheel. The Taldan cavalry cannot maneuver. The Taldan army is surrounded from an army attacking on all sides, trapped by a riverside by an enemy it cannot see, and its knights cannot charge; all they can do is withdraw to the inside of the shelter of soldiers and shoot arrows. The Taldan army begins to make a D, as it bends under the pressure; one line against the river and the rest a half-circle pressing inwards further and further as men take one step back and then another, their formations jammed closer and closer together as the gaps in their line meet each other and the sergeants desperately try to restore order. 

And then, of course, the undead come out of the river.

There is, actually, a Wish wording to turn a whole river to holy water, but she's not here to show off, even if she is zipping around mowing down undead by the thousands with Holy Word after Holy Word after Holy Word. The thing that matters is to give her a shot at Tar-Baphon before he knows he needs to fear her.

Iomedae flies, and heals herself until all of her internal organs are wholly internal where they belong, and is barely faster at healing than they are at shooting her. Once she's hovering in the top layers of the fog the enemy mostly stops firing on her; they presumably can't see through it either. She can see them; most of the fog is illusion, and most of the illusions don't fool her.

 That is really and truly far too many undead. How many necromancers would you need to have that many undead. From above it's clear that the army's surrounded, and that they can't fight effectively surrounded, and that they mostly can't see through the illusions to see where the undead have been thinned out, where there's a break in the enemy lines -

- Aroden, reveal the whole truth to us -

Aroden has higher priorities, right now. The people working for the last fellow to go toe-to-toe with Aroden, on the other hand - 

- Iomedae is really not their priority. She's just some paladin. There's lots of paladins. They kill paladins all the time. The astral deva throwing Holy Word around like it's nothing, though, she needs to go. A black-cloaked skeleton surrounded by ghostly dancers whirling through the army's lines (and, around them, dragging men to join them until their life fades) ceases its work, turns to dart through the sky, sightless eyes searching. The dessicated corpses of great snakes surge out of the fog into the sky, seeking, life, life, ever life, and from the mirrored helmets of the hippogriff knights emerge gore-dripping bones to throw them to the ground and wrench on their horses reins, seeking the deva. Ghoul wizards fly invisibly through the air, Glitterdust spells at the ready, for they know the smell of an angel's flesh. There is always a reward to be had for destroying a great angel, one from Urgathoa as much as from Tar-Baphon, for those who can accomplish it. 

(And, seeing through the fog that they know is an illusion, the best archers of Tar-Baphon's army wait, bows at the ready, for a sign that their target can be seen.)

Which is not to say that they have it all their own way. The dwarves hack undead down by the dozen, axe and shield in accord behind their heavy armor. Web and Grease spells are simple and easy-to-cast and terribly, terribly effective against the armies, and all the most deadly undead that try to come near the battlefield will be brought down by rain after rain of arrows.

The enemy scouts' main problem is that Arazni's very very fast, and zigzagging unpredictably from each Holy Word to the next, and that few of the scouts can actually survive being near her, and that her protective aura functions as a lesser globe of invulnerability so they have to waste fourth circle spell slots if they want to throw out Glitterdusts. Or, those are the problems that they know they have; on top of those, there are some other barriers to their striking successfully at her, including actually a complete immunity to Glitterdust and Invisibility Purge and Dispel Magic and Greater Dispel Magic and Mage's Disjunction - but those she does not wish to be discovered.

Nor does she wish Tar-Baphon to arrive here intending to kill one particular stubborn astral deva; better if he assumes that already accomplished. And so, after a few more Holy Words have scoured Tar-Baphon's scouts further -

Time Stop. Summon Monster IX. Greater Invisibility, welcome to the team please take this shrunken prop I made with Minor Creation ten minutes ago -

And shortly after time resumes, a Glitterdust catches a summoned astral deva on the wing, and as that summoned astral deva dies before a barrage of specialized arrows from accomplished archers Arazni speaks the command word to unshrink the prop, and the battered body of an astral deva falls out of the sky and into the forest. 

(That's an opportunity for a couple of retaliatory Fireballs at the accomplished archers, of course, now that she knows where they are. Arazni's actually really good at Fireballs but these are normal Fireballs, what a shame.)

Arazni switches from Holy Words to Dispel Magics, at least momentarily. The cavalry needs to get out and start actually fighting, and it needs to do that promptly so that Tar-Baphon feels the need to get here in a hurry. On the other side of the river the illusions masking the undead are going to start falling one by one.